UC-NRLF 


vS 


WORLDS 
IN    THE    MAKING 

THE   EVOLUTION    OF    THE   UNIVERSE 


BY 

SVANTE     ARRHENIUS 

IV 

DIRECTOR   OF  THE  PHYSICO-CHEMICAL   NOBEL 
INSTITUTE,    STOCKHOLM 


TRANSLATED    BY 
DR.     H.     BORNS 

ILLUSTRATED 


< 


NEW    YORK    AND    LONDON 

HARPER  &  BROTHERS   PUBLISHERS 

M  C  M  V  I  I  I 


GENERAL 


c  6X 


U 


Copyright,  1908,  by  HARPER  &  BROTHERS. 

All  rights  reserved. 
Published  March,  1908. 


TABLE    OF    CONTENTS 


I.  VOLCANIC   PHENOMENA  AND   EARTHQUAKES     .     .       1 

Destruction  caused  by  volcanism  and  by  earthquakes. — 
Different  kinds  of  volcanoes. — Vesuvius. — Products  of  erup- 
tion.—  Volcanic  activity  diminishing.  —  Structure  of  vol- 
canoes.— Geographical  distribution  of  volcanoes. — Tempera- 
ture in  the  interior  of  the  earth.  —  Significance  of  water 
for  volcanism. — Composition  of  the  earth's  interior. — Geo- 
graphical distribution  of  earthquakes. — Fissures  in  the 
earth's  crust. — Groups  of  earthquakes. — Waves  in  the  sea 
and  in  the  air  accompanying  earthquakes. — Their  connec- 
tion with  volcanism. — Systems  of  fissures. — Seismograms. 

II.  THE   CELESTIAL    BODIES,    IN    PARTICULAR    THE 

EARTH,  AS  ABODES  OF  LIVING  BEINGS     ...     39  ^ 

Manifold  character  of  the  worlds. — The  earth  probably  at 
first  a  ball  of  gases. — Formation  of  the  earth  crust  and  its 
rapid  cooling. — Balance  between  heat  received  and  heat 
lost'  by  radiation. — Life  already  existing  on  the  earth  for 
a  milliard  of  years. — The  waste  of  solar  heat. — Temperature 
and  habitability  of  the  planets. — Heat-preserving  influence 
of  the  atmosphere. — Significance  of  carbon  dioxide  in  the 
atmosphere  —  Warm  and  cold  geological  ages.  —  Fluctua- 
tions in  the  percentage  of  carbon  dioxide  of  the  air. — Com- 
bustion, decay,  and  growth. — Atmospheric  oxygen. — Vege- 
table life  more  ancient  than  animal  life. — The  atmospheres 
of  planets. — Chances  of  an  improvement  in  the  climate. 

III.  RADIATION   AND  CONSTITUTION   OF  THE  SUN  .     64 

Stability  of  the  solar  system. — Losses  and  possible  gains 
of  heat  by  the  sun. — Theses  of  Mayer  and  of  Helmholtz. — 
Temperatures  of  the  white,  yellow,  and  reddish  stars,  and 
of  the  sun. — Sun-spots  and  sun  faculse. — Prominences. — 


TABLE  OF  CONTENTS 

Spectra  of  the  parts  of  the  sun. — Temperature  of  the  sun. — 
The  interior  of  the  sun. — Its  composition  according  to  the 
mechanical  theory  of  heat. — The  losses  of  heat  by  the  sun 
probably  covered  by  the  enormous  solar  energy. 

IV.  THE  RADIATION   PRESSURE 94 

Newton's  law  of  gravitation. — Kepler's  observation  of 
comets'  tails. — The  thesis  of  Euler. — Proof  of  Maxwell. — 
The  radiation  pressure. — Electric  charges  and  condensa- 
tion.— Comets'  tails  and  radiation  pressure. — Constituents 
and  properties  of  comets'  tails. — Weight  of  the  solar  corona. 
— Loss  and  gain  of  matter  by  the  sun. — Nature  of  meteor- 
ites.— Electric  charge  of  the  sun. — Electrons  drawn  into  the 
sun. — Magnetic  properties  of  the  sun  and  appearance  of  the 
corona. — Constituents  of  the  meteors. — Nebulae  and  their 
heat  and  light. 

V.  THE  SOLAR  DUST  IN  THE  ATMOSPHERE.     POLAR 

LIGHTS    AND    THE    VARIATIONS     OF    TERRES- 
TRIAL MAGNETISM 118 

The  supply  of  dust  from  the  sun  rather  insignificant. — 
Polarization  of  the  light  of  the  sky. — The  upper  clouds. — 
Different  kinds  of  aurorse. — Their  connection  with  the 
corona  of  the  sun. — Polar  lights  and  sun-spots — Periodicity 
of  polar  lights. — Polar  lights  and  magnetic  disturbances. — 
Velocity  of  solar  dust. — -Fixation  of  atmospheric  nitrogen.— 
The  Zodiacal  Light. 

VI.  END  OF  THE  SUN.— ORIGIN   OF   NEBULA  ...  148 

The  extinction  of  the  sun. — Collision  between  two  celestial 
bodies. — The  new  star  in  Perseus. — Formation  of  nebulae. — 
The  appearance  of  nebulae. — The  nebulae  catch  wandering 
meteors  and  comets. — The  ring  nebula  in  Lyra. — Variable 
stars. — Eta  in  Argus. — Mira  Ceti. — Lyra  and  Algol  stars. — 
Evolution  of  the  stars. 

VII.  THE  NEBULAR   AND   THE   SOLAR   STATES     .     .191 

The  energy  of  the  universe. — The  entropy  of  the  universe. — 
The  entropy  increases  in  the  suns,  but  decreases  in  the 
nebulae. — Temperature  and  constitution  of  the  nebulae. — 
Schuster's  calculations  of  the  condition  of  a  celestial  body 
consisting  of  gases. — Action  of  the  loss  of  heat  on  nebulae 
iv 


TABLE   OF   CONTENTS 

and  on  suns.  —  Development  of  a  rotating  nebula  into  a 
planetary  system.  —  The  hypothesis  of  Kant-Laplace.  —  Ob- 
jections to  it.  —  The  views  of  Chamberlin  and  Moulton.  — 
The  radiation  pressure  balances  the  effect  of  Newtonian 
gravitation.  —  The  emission  of  gases  from  the  nebulae  bal- 
ances the  waste  of  heat  characteristic  to  the  solar  systems. 

VIII.     THE    SPREADING     OF     LIFE     THROUGH    THE 

UNIVERSE     ...     ............       212 

Stability  of  the  species.  —  Theory  of  mutation.  —  Sponta- 
neous generation.  —  Bathybius.  —  Panspermia.  —  The  stand- 
points of  Richter,  Ferdinand  Cohn,  and  Lord  Kelvin.  —  The 
radiation  pressure  enables  spores  to  escape.  —  The  effect  of 
strong  sunlight  and  of  cold  on  the  germinating  power.  — 
Transport  of  spores  through  the  atmosphere  into  universal 
space  and  through  it  to  other  planets.  —  General  conclusions. 

EXPLANATION   OF   ABBREVIATIONS,   ETC. 

The  temperatures  are  stated  in  degrees  centigrade  (°  C.),  either  on 
the  Celsius  scale,  on  which  the  freezing-point  of  water  is  O°,  or  on 
the  absolute  scale,  whose  zero  lies  273  degrees  below  the  freezing- 
point  of  water,  at  —273°  C.  The  equivalent  temperatures  on  the 
Fahrenheit  scale  (freezing-point  of  water  32°  F.)  are  added  in 
brackets  (°  F.). 

1  metre  (m.)  =10  decimetres  (dm.)  =100  centimetres  (cm.)  =1000 


millimetres  (mm.)  =3.28  ft.;   1  kilometre  (km.)  =1000  metres  (rn.)  ^v 
=  1.6  miles  ;   1  mile  =0.62  kilometres  (km.). 

Light  travels  in  yacuo  at  the  rate  of  300,000  km.  (nearly  200,000 
miles)  per  second. 


ILLUSTRATIONS 


PAGE 


1.  VESUVIUS,     AS     SEEN     FROM     THE     ISLAND     OP     NISIDA,      IN 

MODERATE    ACTIVITY      ........       ....  2 

2.  ERUPTION  OF  VESUVIUS  IN  1882    ......       ....  4 

3.  ERUPTION  OF  VESUVIUS  IN  1872    ..........  6 

4.  PHOTOGRAPH     OF     VESUVIUS,      1906.       CHIEFLY     CLOUDS      OF 

ASHES     ...........       .       .....  8 

5.  BLOCK  LAVA  ON  MAUNA  LOA       ..........      10 

6.  THE  EXCELSIOR  GEYSER  IN  YELLOWSTONE  PARK,  U.  S.  A. 

REMNANT  OF   POWERFUL  VOLCANIC   ACTIVITY   IN  THE 
TERTIARY  AGE    ..............       11 

7.  MATO  TEPEE  IN  WYOMING,  U.   S.  A.    TYPICAL  VOLCANIC 

"NECK"     ................       12 

8.  CLEFTS  FILLED  WITH  LAVA  AND  VOLCANIC  CONE  OF  ASHES, 

TOROWHEAP  CANON,  PLATEAU  OF  COLORADO  ....       13 

9.  THE  KILAUEA  CRATER  ON  HAWAII  .........      15 

10.  CHIEF  EARTHQUAKE  CENTRES,  ACCORDING  TO  THE  BRITISH 

ASSOCIATION  COMMITTEE      .........    .      22 

11.  CLEFTS  IN  VALENTIA  STREET,  SAN  FRANCISCO,  AFTER  THE 

EARTHQUAKE  OF  1906     ...........      25 

12.  SAND  CRATERS  AND  FISSURES,  PRODUCED  BY  THE  CORINTH 

EARTHQUAKE  OF  1861.    IN  THE  WATER,  BRANCHES  OF 
FLOODED  TREES       .............       27 

13.  EARTHQUAKE  LINES  IN  LOWER  AUSTRIA  .......      30 

14.  LIBERTY  BUILDING  OF  LELAND  STANFORD  JUNIOR  UNIVER- 

SITY, IN  CALIFORNIA,  AFTER  THE  EARTHQUAKE  OF  1906      32 

15.  EARTHQUAKE  LINES  IN  THE  TYRRHENIAN  DEPRESSION    .    .      34 

vii 


ILLUSTRATIONS 

FIG.  PAGE 

16.  SEISMOGRAM    RECORDED    AT    SHIDE,    ISLE    OF    WIGHT,    ON 

AUGUST  31,  1898 35 

17.  PHOTOGRAPH  OF  THE  SURFACE  OF  THE  MOON,  IN  THE  VICIN- 

ITY OF  THE  CRATER  OF  COPERNICUS 62 

18.  SUN-SPOT  GROUP  AND  GRANULATION  OF  THE  SUN    ...  74 

19.  PART  OF  THE  SOLAR  SPECTRUM  OF  JANUARY  3,  1872     .     .  75 

20.  METALLIC  PROMINENCES  IN  VORTEX  MOTION 76 

21.  FOUNTAIN-LIKE  METALLIC  PROMINENCES 76 

22.  QUIET  PROMINENCES  OF  SMOKE-COLUMN  TYPE  .....  77 

23.  QUIET  PROMINENCES,  SHAPE  OF  A  TREE 77 

24.  DIAGRAM  ILLUSTRATING  THE  DIFFERENCES  IN  THE  SPECTRA  ' 

OF  SUN-SPOTS  AND  OF  THE  PHOTOSPHERE 78 

25.  SPECTRUM  OF  A  SUN-SPOT,  THE  CENTRAL  BAND  BETWEEN 

THE  TWO  PORTIONS  OF  THE  PHOTOSPHERES  PECTRUM  .  78 

26.  THE  GREAT  SUN-SPOT  OF  OCTOBER  9,  1903 79 

27.  THE  GREAT  SUN-SPOT  OF  OCTOBER  9,  1903  ......  80 

28.  THE  GREAT  SUN-SPOT  OF  OCTOBER  9,  1903 81 

29.  THE  GREAT  SUN-SPOT  OF  OCTOBER  9,  1903 82 

30.  PHOTOGRAPH  OF  THE  SOLAR  CORONA  OF  1900 83 

31.  PHOTOGRAPH  OF  THE  SOLAR  CORONA  OF  1870 84 

32.  PHOTOGRAPH  OF  THE  SOLAR  CORONA  OF  1898 85 

33.  PHOTOGRAPH  OF  ROERDAM's  COMET  (1893  II.),  SUGGESTING 

SEVERAL  STRONG  NUCLEI  IN  THE  TAIL 100 

34.  PHOTOGRAPH  OF  SWIFT'S  COMET  (1892  I.) 101 

35.  DONATI'S  COMET  AT  ITS  GREATEST  BRILLIANCY  IN  1858     .  102 

36.  IMITATION  OF  COMETS'  TAILS 104 

37.  GRANULAR  CHONDRUM  FROM  THE   METEORITE   OF  SEXES. 

ENLARGEMENT  1  :  70 109 

38.  ARCH-SHAPED   AURORA   BOREALIS,   OBSERVED   BY   NORDEN- 

SKIOLD    DURING    THE   WINTERING    OF    THE    VEGA    IN 

BERING  STRAIT,  1879 124 

39.  AURORA  BOREALIS,  WITH  RADIAL  STREAMERS 125 

40.  AURORA    WITH    CORONA,   OBSERVED    BY   GYLLENSKIOLD    ON 

SPITZBERGEN,    1883 126 

V  viii 


ILLUSTRATIONS 

FIO.  PAGE 

41.  POLAR-LIGHT  DRAPERIES,  OBSERVED  IN  FINNMARKEN,  NORTH- 

ERN NORWAY 127 

42.  CURVE  OF  MAGNETIC  DECLINATION  AT  KEW,  NEAR  LONDON, 

ON  NOVEMBER  15  AND  16,  1905 138 

43.  CURVE  OF  HORIZONTAL  INTENSITY  AT  KEW  ON  NOVEMBER 

15  AND  16,  1905 139 

44.  ZODIACAL  LIGHT  IN  THE  TROPICS 146 

45.  SPECTRUM  OF  NOVA  AURIGA,  1892 .  154 

46.  DIAGRAM  INDICATING  THE   CONSEQUENCES   OF  A  COLLISION 

BETWEEN  TWO  EXTINCT  SUNS .157 

47.  SPIRAL  NEBULA  IN  THE  CANES  VENATICI 159 

48.  SPIRAL  NEBULA  IN  THE  TRIANGLE v  .     .    .  161 

49.  THE  GREAT  NEBULA  IN  ANDROMEDA    . 163 

50.  RING-SHAPED  NEBULA  IN  LYRA 164 

51.  CENTRAL  PORTION  OF  THE  GREAT  NEBULA  IN  ORION  .     .     .  165 

52.  NEBULAR  STRLE  IN  THE  STARS  OF  THE  PLEIADES     ....  167 

53.  NEBULAR  STRLE  IN  THE  SWAN 169 

54.  NEBULA  AND  STAR  RIFT  IN  THE  SWAN,  IN  THE  MILKY  WAY  171 

55.  GREAT  NEBULA  NEAR  RHO,  IN  OPHIUCHUS 172 

56.  STAR  CLUSTER  IN  HERCULES 173 

57.  STAR'  CLUSTER  IN  PEGASUS 175 

58.  CONE-SHAPED  STAR  CLUSTER  IN  GEMINI 176 

59.  COMPARISON  OF  SPECTRA  OF  STARS  OF  CLASSES  2,  3,  4     .  185 

60.  COMPARISON  OF  SPECTRA  OF  STARS  OF  CLASSES  2,  3,  4     .  186 


PREFACE 

WHEN,  more  than  six  years  ago,  I  was  writing  my 
Treatise  of  Cosmic  Physics,  I  found  myself  confronted 
with  great  difficulties.  The  views  then  held  would  not 
explain  many  phenomena,  and  they  failed  in  particular 
in  cosmogonic  problems.  The  radiation  pressure  of 
light,  which  had  not,  so  far,  been  heeded,  seemed  to% 
give  me  the  key  to  the  elucidation  of  many  obscure 
problems,  and  I  made  a  large  use  of  this  force  in  dealing 
with  those  phenomena  in  my  treatise. 

The  explanations  which  I  tentatively  offered  could,  of 
course,  not  claim  to  stand  in  all  their  detail;  yet  the 
scientific  world  received  them  with  unusual  interest  and 
benevolence.  Thus  encouraged,  I  tried  to  solve  more  of 
the  numerous  important  problems,  and  in  the  present 
volume  I  have  added  some  further  sections  to  the  com- 
plex of  explanatory  arguments  concerning  the  evolution 
of  the  Universe.  The  foundation  to  these  explanations 
was  laid  in  a  memoir  which  I  presented  to  the  Academy 
of  Sciences  at  Stockholm  in  1900.  The  memoir  was  soon 
afterwards  printed  in  the  Physikalische  Zeitschrift,  and 
the  subject  was  further  developed  in  my  Treatise  of 
Cosmic  Physics. 

It  will  be  objected,  and  not  without  justification,  that 
scientific  theses  should  first  be  discussed  and  approved  of 
in  competent  circles  before  they  are  placed  before  the 
public.  It  cannot  be  denied  that,  if  this  condition  were 

xi 


PREFACE 

to  be  fulfilled,  most  of  the  suggestions  on  cosmogony  that 
have  been  published  would  never  have  been  sent  to  the 
compositors;  nor  do  I  deny  that  the  labor  spent  upon  their 
publication  might  have  been  employed  for  some  better 
purpose.  But  several  years  have  elapsed  since  my  first 
attempts  in  this  direction  were  communicated  to  scien- 
tists. My  suggestions  have  met  with  a  favorable  recep- 
tion, and  I  have,  during  these  years,  had  ample  op- 
portunity carefully  to  re-examine  and  to  amend  my 
explanations.  I  therefore  feel  justified  in  submitting 
my  views  to  a  larger  circle  of  readers. 

The  problem  of  the  evolution  of  the  Universe  has  al- 
ways excited  the  profound  interest  of  thinking  men.  And 
it  will,  without  doubt,  remain  the  most  eminent  among 
all  the  questions  which  do  not  have  any  direct,  practical 
bearing.  Different  ages  have  arrived  at  different  solu- 
tions to  this  great  problem.  Each  of  these  solutions  re- 
flected the  stand-point  of  the  natural  philosophers  of  its 
time.  Let  me  hope  that  the  considerations  which  I  offer 
will  be  worthy  of  the  grand  progress  in  physics  and  chem- 
istry that  has  marked  the  close  of  the  nineteenth  and 
the  opening  of  the  twentieth  century. 

Before  the  indestructibility  of  energy  was  understood, 
cosmogony  merely  dealt  with  the  question  how  matter 
could  have  been  arranged  in  such  a  manner  as  to  give 
rise  to  the  actual  worlds.  The  most  remarkable  con- 
ception of  this  kind  we  find  in  Herschel's  suggestion  of 
the  evolution  of  stellar  nebulae,  and  in  the  thesis  of  La- 
place concerning  the  formation  of  the  solar  system  out 
of  the  universal  nebula.  Observations  more  and  more 
tend  to  confirm  Herschel's  view.  The  thesis  of  Laplace, 
for  a  long  time  eulogized  as  the  flower  of  cosmogonic 
speculations,  has  more  and  more  had  to  be  modified.  If 
we  attempt,  with  Kant,  to  conceive  how  wonderfully 

xii 


PREFACE 

organized  stellar  systems  could  originate  from  absolute 
chaos,  we  shall  have  to  admit  that  we  are  attacking  a 
problem  which  is  insoluble  in  that  shape.  There  is  a 
contradiction  in  those  very  attempts  to  explain  the  origin 
of  the  Universe  in  its  totality,  as  Stallo1  emphasizes: 
"  The  only  question  to  which  a  series  of  phenomena  gives 
legitimate  rise  relates  to  their  filiation  and  interdepend- 
ence." I  have,  therefore,  only  endeavored  to  show  how 
nebulae  may  originate  from  suns  and  suns  from  nebulae; 
and  I  assume  that  this  change  has  always  been  proceed- 
ing as  it  is  now. 

The  recognition  of  the  indestructibility  of  energy 
seemed  to  accentuate  the  difficulties  of  the  cosmogonic 
problems.  The  theses  of  Mayer  and  of  Helmholtz,  on  the 
manner  in  which  the  Sun  replenishes  its  losses  of  heat, 
have  had  to  be  abandoned.  My  explanation  is  based 
upon  chemical  reactions  in  the  interior  of  the  Sun  in 
accordance  with  the  second  law  of  thermodynamics. 
The  theory  of  the  "degradation"  of  energy  appeared  to 
introduce  a  still  greater  difficulty.  That  theory  seems 
to  lead  to  the  inevitable  conclusion  that  the  Universe 
is  tending  towards  the  state  which  Clausius  has  desig- 
nated as  "  Wdrme  Tod"  (heat  death),  when  all  the  energy 
of  the  Universe  will  uniformly  be  distributed  through 
space  in  the  shape  of  movements  of  the  smallest  particles. 
That  would  imply  an  absolutely  inconceivable  end  of  the 
development  of  the  Universe.  The  way  out  of  this  diffi- 
culty which  I  propose  comes  to  this:  the  energy  is  " de- 
graded" in  bodies  which  are  in  the  solar  state,  and  the 
energy  is  "elevated,"  raised  to  a  higher  level,  in  bodies 
which  are  in  the  nebular  state. 

Finally,  I  wish  to  touch  upon  one  cosmogonical  ques- 

1  Stallo  :  Concepts  and  Theories  of  Modern  Physics.  London,  1900, 
p.  276. 

xiii 


PREFACE 

tion  which  has  recently  become  more  actual  than  it  used 
to  be.  Some  kind  of  "spontaneous  generation,"  origina- 
tion of  life  from  inorganic  matter,  had  been  acquiesced  in. 
But  just  as  the  dreams  of  a  spontaneous  generation  of 
energy — i.e.,  of  a  perpetuum mobile — have  been  dispelled 
by  the  negative  results  of  all  experiments  in  that  direction, 
just  in  the  same  way  we  shall  have  to  give  up  the  idea  of 
a  spontaneous  generation  of  life  after  all  the  repeated 
disappointments  in  this  field  of  investigation.  As 
Helmholtz1  says,  in  his  popular  lecture  on  the  growth 
of  the  planetary  system  (1871) :  "  It  seems  to  me  a  per- 
fectly just  scientific  procedure,  if  we,  after  the  failure  of 
all  our  attempts  to  produce  organisms  from  lifeless  matter, 
put  the  question,  whether  life  has  had  a  beginning  at 
all,  or  whether  it  is  not  as  old  as  matter,  and  whether 
seeds  have  not  been  carried  from  one  planet  to  another 
and  have  developed  everywhere  where  they  have  fallen 
on  a  fertile  soil." 

This  hypothesis  is  called  the  hypothesis  of  panspermia, 
which  I  have  modified  by  combining  it  with  the  thesis 
of  the  radiation  pressure. 

My  guiding  principle  in  this  exposition  of  cosmogonic 
problems  has  been  the  conviction  that  the  Universe  in 
its  essence  has  always  been  what  it  is  now.  Matter,  en- 
ergy, and  lifs  have  only  varied  as  to  shape  and  position 
in  space. 

THE  AUTHOB. 

STOCKHOLM,  December,  1907. 

1  Helmholtz,  Popular^  Wissenschaftliche  Vortrage.  Braunschweig, 
1876,  vol.  iii.,  p.  101. 


WORLDS    IN   THE   MAKING 


VOLCANIC   PHENOMENA    AND   EARTHQUAKES 
The  Interior  of  the  Earth 

THE  disasters  which  have  recently  befallen  the  flour- 
ishing settlements  near  Vesuvius  and  in  California  have 
once  more  directed  the  attention  of  mankind  to  the  terrific 
forces  which  manifest  themselves  by  volcanic  eruptions 
and  earthquakes. 

The  losses  of  life  which  have  been  caused  in  these  two 
last  instances  are,  however,  insignificant  by  comparison 
with  those  which  various  previous  catastrophes  of  this 
kind  have  produced.  The  most  violent  volcanic  eruption 
of  modern  times  is  no  doubt  that  of  August  26  and  27, 
1883,  by  which  two-thirds  of  the  island  of  Krakatoa,  33 
square  kilometres  (13  square  miles)  in  area,  situated  in 
the  East  Indian  Archipelago,  were  blown  into  the  air. 
Although  this  island  was  itself  uninhabited,  some  40,000 
people  perished  on  that  occasion,  chiefly  by  the  ocean 
wave  which  followed  the  eruption  and  which  caused 
disastrous  inundations  in  the  district.  Still  more  terrible 
was  the  destruction  wrought  by  the  Calabrian  earth- 
quake of  February  and  March,  1783,  which  consisted  of 

1 


WORLDS   IN  THE   MAKING 


several  earthquake  waves.  The  large  town  of  Messina 
was  destroyed  on  February  5th,  and  the  number  of 
people  killed  by  this  event  has  been  estimated  at  100,000. 
The  same  region,  especially  Calabria,  has,  moreover,  fre- 
quently been  visited  by  disastrous  earthquakes — again 
in  1905  and  1907.  Another  catastrophe  upon  which 
history  dwells,  owing  to  the  loss  of  life  (not  less  than 
90,000),  was  the  destruction  of  the  capital  of  Portugal 
on  November  1,  1755.  Two-thirds  of  the  human  lives 
which  this  earthquake  claimed  were  destroyed  by  a  wave 
5  m.  in  height  rushing  in  from  the  sea. 

Vesuvius  is  undoubtedly  the  best  studied  of  all  vol- 
canoes. During  the  splendor  of  Rome  this  mountain  was 
quite  peaceful — known  as  an  extinct  volcanic  cone  so  far 
as  history  could  be  traced  back.  On  the  extraordinari- 
ly fertile  soil  about  it  had  arisen  big  colonies  of  such 
wealth  that  the  district  was  called  Great  Greece  (Grsecia 


Fig.  1. — Vesuvius,  as  seen  from  the  Island  of  Nisida,  in 
moderate  activity 

Magna).  Then  came,  in  the  year  79  A. D.,  the  devastating 
eruption  which  destroyed,  among  others,  the  towns  of 
Herculaneum  and  Pompeii.  The  volumes  of  gas,  rushing 
forth  with  extreme  violence  from  the  interior  of  the 
earth,  pushed  aside  a  large  part  of  the  volcanic  cone 
whose  remnant  is  now  called  Monte  Somma,  and  the 
falling  masses  of  ashes,  mixed  with  streams  of  lava, 

2 


VOLCANIC  PHENOMENA  AND   EARTHQUAKES 

built  up  the  new  Vesuvius.  This  mountain  has  repeat- 
edly changed  its  appearance  during  later  eruptions, 
and  was  provided  with  a  new  cone  of  ashes  in  the 
year  1906.  The  outbreak  of  the  year  79  was  succeeded 
by  new  eruptions  in  the  years  203,  472,  512,  685,  993, 
1036,  1139,  1500,  1631,  and  1660,  at  quite  irregular 
intervals.  Since  that  time  Vesuvius  has  been  in  al- 
most uninterrupted  activity,  mostly,  however,  of  a  harm- 
less kind,  so  that  only  the  cloud  of  smoke  over  its  crater 
indicated  that  the  internal  glow  was  not  yet  extinguished. 
Very  violent  eruptions  took  place  in  the  years  1794, 
1822,  1872,  and  1906. 

Other  volcanoes  behave  quite  differently  from  these 
violent  volcanoes,  and  do  hardly  any  noteworthy  dam- 
age. Among  these  is  the  crater-island  of  Stromboli,  situ- 
ated between  Sicily  and  Calabria.  This  volcano  has  been 
in  continuous  activity  for  thousands  of  years.  Its  erup- 
tions succeed  one  another  at  intervals  ranging  from  one 
minute  to  twenty  minutes,  and  its  fire  serves  the  sailors 
as  a  natural  light-house.  The  force  of  this  volcano  is,  of 
course,  unequal  at  different  periods.  In  the  summer  of 
1906  it  is  said  to  have  been  in  unusually  violent  activity. 
Very  quiet,  as  a  rule,  are  the  eruptions  of  the  great  vol- 
canoes on  Hawaii. 

Foremost  among  the  substances  which  are  ejected  from 
volcanoes  is  water  vapor.  The  cloud  floating  above  the 
crater  is,  for  this  reason,  the  surest  criterion  of  the  activity 
of  the  volcano.  Violent  eruptions  drive  the  masses  of 
steam  up  into  the  air  to  heights  of  8  km.  (5  miles),  as  the 
illustrations  (Figs.  1  to  4)  will  show. 

The  height  of  the  cloud  may  be  judged  from  the  height 
of  Vesuvius,  1300  metres  (nearly  4300  ft.)  above  sea- 
level.  The  illustration  on  page  4  (Fig.  2)  is  a  repro- 
duction of  a  drawing  by  Poulett  Scrope,  representing  the 


WORLDS    IN   THE   MAKING 

Vesuvius  eruption  of  the  year  1822.  There  seems  to 
have  been  no  wind  on  this  day;  the  masses  of  steam 
formed  a  cloud  of  a  regular  shape  which  reminds  us  of  a 
pine-tree.  According  to  the  description  of  Plinius,  the 
cloud  noticed  at  the  eruption  of  Vesuvius  in  the  year  79 


Fig.  2. — Eruption  of  Vesuvius  in  1882.     (After  a 
contemporaneous  drawing  by  Poulett  Scrope) 

must  nave  been  of  the  same  kind.  When  the  air  is  not  so 
calm  the  cloud  assumes  a  more  irregular  shape  (Fig.  3). 
Clouds  which  rise  to  such  elevations  as  we  have  spoken 
of  are  distinguished  by  strong  electric  charges.  The 

4 


VOLCANIC  PHENOMENA  AND  EARTHQUAKES 

vivid  flashes  of  lightning  which  shoot  out  of  the  black 
clouds  add  to  the  terror  of  the  awful  spectacle. 

The  rain  which  pours  down  from  this  cloud  is  often 
mixed  with  ashes  and  is  as  black  as  ink.  The  ashes  have 
a  color  which  varies  between  light -gray,  yellow -gray, 
brown,  and  almost  black,  and  they  consist  of  minute 
spherules  of  lava  ejected  by  the  force  of  the  gases  and 
rapidly  congealed  by  contact  with  the  air.  Larger  drops 
of  lava  harden  to  volcanic  sand — the  so-called  "lapilli" 
(that  is,  little  stones),  or  to  "bombs,"  which  are  often 
furrowed  by  the  resistance  offered  by  the  air,  and  turn 
pear-shaped.  These  solid  products,  as  a  rule,  cause  the 
greatest  damage  due  to  volcanic  eruptions.  In  the  year 
1906  the  weight  of  these  falling  masses  (Fig.  4)  crushed 
in  the  roofs  of  houses.  A  layer  of  ashes  7  m.  (23  ft.)  in 
thickness  buried  Pompeii  under  a  protective  crust  which 
had  covered  it  up  to  days  of  modern  excavations.  The 
fine  ashes  and  the  muddy  rain  clung  like  a  mould  of 
plaster  to  the  dead  bodies.  The  mud  hardened  after- 
wards into  a  kind  of  cement,  and  as  the  decomposition 
products  of  the  dead  bodies  were  washed  away,  the 
moulds  have  provided  us  with  faithful  casts  of  the  ob- 
jects that  had  once  been  embedded  in  them.  When  the 
ashes  fall  into  the  sea,  a  layer  of  volcanic  tuffa  is  formed 
in  a  similar  manner,  which  enshrines  the  animals  of  the 
sea  and  algse.  Of  this  kind  is  the  soil  of  the  Campagna 
Felice,  near  Naples.  Larger  lumps  of  solid  stones  with 
innumerable  bubbles  of  gases  float  as  pumice-stone  on 
the  sea,  and  are  gradually  ground  down  into  volcanic 
sand  by  the  action  of  the  waves.  The  floating  pum- 
ice-stone has  sometimes  become  dangerous  or,  at  any 
rate,  an  obstacle  to  shipping,  through  its  large  masses;  that 
was,  at  least,  the  case  with  the  Krakatoa  eruption  of 
1883. 

5 


WORLDS  IN   THE   MAKING 

Among  the  gases  which  are  ejected  in  addition  to 
water  vapor,  carbonic  acid  should  be  mentioned  in  the 
first  instance;  also  vapors  of  sulphur  and  sulphuretted 
hydrogen,  hydrochloric  acid,  and  chloride  of  ammonium, 
as  well  as  the  chlorides  of  iron  and  copper,  boric  acid, 
and  other  substances.  A  large  portion  of  these  bodies 
is  precipitated  on  the  edges  of  the  volcano,  owing  to 


Fig.  3. — Eruption  of  Vesuvius  in  1872.     (After  a  photograph. ) 

the  sudden  cooling  of  the  gases.  The  more  volatile  con- 
stituents, such  as  carbonic  acid,  sulphuretted  hydrogen, 
and  hydrochloric  acid,  may  spread  over  large  areas,  and 
destroy  all  living  beings  by  their  heat  and  poison.  It 
was  these  gases,  for  example,  which  caused  the  awful 
devastation  at  St.  Pierre,  where  30,000  human  lives  were 
destroyed  on  May  8,  1902,  by  the  eruption  of  Mont  Pelee. 
The  ejection  of  hydrogen  gas,  which,  on  emerging  from 

6 


VOLCANIC  PHENOMENA  AND  EARTHQUAKES 

the  lava,  is  burned  to  water  by  the  oxygen  of  the  air,  has 
been  observed  in  the  crater  of  Kilauea. 

The  ashes  of  the  volcanoes  are  sometimes  carried  to 
vast  distances  by  the  air  currents — e.  g.,  from  the  west- 
ern coast  of  South  America  to  the  Antilles;  from  Iceland 
to  Norway  and  Sweden;  from  Vesuvius  (1906)  to  Hoi- 
stein.  Best  known  in  this  respect  is  the  eruption  of 
the  Krakatoa,  which  drove  the  fine  ashes  up  to  an  ele-- 
vation  of  30  km.  (18  miles).  The  finest  particles  of 
these  ashes  were  slowly  carried  by  the  winds  to  all  parts 
of  the  earth,  where  they  caused,  during  the  following 
two  years,  the  magnificent  sunrises  and  sunsets  which 
were  spoken  of  as  "the  red  glows."  This  glow  was  also 
observed  in  Europe  after  the  eruption  of  Mont  Pelee. 
The  dust  of  Krakatoa  further  supplied  the  material  for 
the  so-called  "luminous  clouds  of  the  night/'  which  were 
seen  in  the  years  1883  to  1892  floating  at  an  elevation 
of  about  80  km.  (50  miles),  and  hence  illuminated  by  the 
light  of  the  sun  long  after  sunset. 

The  crater  of  Kilauea,  on  the  high  volcano  of  Mauna  Loa, 
in  Hawaii — this  volcano  is  about  of  the  same  height  as 
Mont  Blanc — has  excited  special  interest.  The  crater  forms 
a  large  lake  of  lava  having  an  area  of  about  12  sq.  km.  (near- 
ly 5  sq.  miles),  which,  however,  varies  considerably  with 
time.  The  lava  boiling  at  red  glow  is  constantly  emitting 
masses  of  gas  under  slight  explosions,  spurting  out  fiery 
fountains  to  a  height  of  20  in.  (65  ft.)  into  the  air.  Here 
and  there  lava  flows  out  from  crevices  in  the  wall  of  the 
crater  down  the  slope  of  the  mountain,  until  the  surface 
of  the  lake  of  lava  has  descended  below  these  cracks. 
As  a  rule,  this  lava  is  of  a  thin  fluid  consistency,  and  it 
spreads,  therefore,  rather  uniformly  over  large  areas. 
Of  a  similar  kind  are  also  the  floods  of  lava  which  are 
sometimes  poured  over  thousands  of  square  kilometres 


WORLDS  IN  THE  MAKING 


Fig.  4. — Photograph  of  Vesuvius,  1906.     Chiefly  clouds  of  ashes 

on  Iceland.  The  so-called  Laid  eruption  of  the  year  1783 
was  of  a  specially  grand  nature.  Though  occurring  in 
an  uninhabited  district,  it  did  a  great  amount  of  dam- 
age. In  the  more  ancient  geological  periods,  especially 
in  the  Tertiary  age,  similar  sheets  of  lava  of  vast  ex- 
tensions have  been  spread  over  England  and  Scotland 
(more  than  100,000  sq.  km.,  roughly,  40,000  sq.  miles); 
over  Deccan,  in  India,  400,000  sq.  kins.  (150,000  sq. 
miles),  up  to  heights  of  2000  m.  (6500  ft.);  and  over 
Wyoming,  Yellowstone  Park,  Nevada,  Utah,  Oregon, 
and  other  districts  of  the  United  States,  as  well  as  over 
British  Columbia. 

In  other  cases    the    slowly  ejected   lava   is    charged 
with  large  volumes  of  gases,  which  escape  when  the  lava 

8 


VOLCANIC  PHENOMENA  AND   EARTHQUAKES 

congeals  and  burst  it  up  into  rough,  unequal  blocks, 
forming  the  so-called  block  lava  (Fig.  5).  The  streams 
of  lava  can  likewise  produce  terrible  devastation  when 
they  descend  into  inhabited  districts;  on  account  of  their 
slow  motion,  they  rarely  cause  loss  of  life,  however. 

Where  the  volcanic  activity  gradually  lessens  or  ceases, 
we  can  still  trace  it  by  the  exhalations  of  gas  and  the 
springs  of  warm  water  which  we  find  in  many  districts 
where,  during  the  Tertiary  age,  powerful  volcanoes  were 
ejecting  their  streams  of  lava.  To  this  class  belong  the 
famous  geysers  of  Iceland,  of  Yellowstone  Park  (Fig. 
6),  and  of  New  Zealand;  also  the  hot  springs  of  Bo- 
hemia, so  highly  valued  therapeutically  (e.  #.,  the  Karls- 
bad Sprudel);  the  Fumaroli  of  Italy,  Greece,  and  other 
countries,  exhaling  water  vapor;  the  Mofettse,  with  their 
exhalations  of  carbonic  acid  (of  frequent  occurrence  in 
the  district  of  the  Eifel  and  on  both  sides  of  the  middle 
Rhine,  in  the  Dogs  Grotto  near  Naples,  and  in  the  Valley 
of  Death  in  Java) ;  the  Solf atara,  exhaling  vapors  of  sul- 
phur— sulphuretted  hydrogen  and  sulphur  dioxide  (they 
are  found  near  Naples  on  the  Phlegrsean  Fields  and  in 
Greece) ;  as  well  as  many  of  the  so-called  mud  volcanoes, 
which  eject  mud,  salt  water,  and  gases  (as  a  rule,  car- 
bonic acid  and  hydrocarbons) — for  example,  the  mud 
volcanoes  near  Parma  and  Modena,  in  Italy,  and  those 
near  Kronstadt,  in  Transylvania. 

The  extinct  volcanoes,  of  which  some,  like  the  Acon- 
cagua, 6970  m.  (22,870  ft.),  in  South  America,  and  the 
Kilimanjaro,  in  Africa,  6010  m.  (19,750  ft.),  rank  among 
the  highest  mountains,  are  exposed  to  a  rapid  destruc- 
tion by  the  rain,  because  they  consist  largely  of  loose 
materials — volcanic  ashes  with  interposed  layers  of  lava. 
Where  these  lava  streams  expand  gradually,  they  pro- 
tect the  ground  underneath  from  erosion  by  water,  and 

9 


WORLDS  IN   THE  MAKING 


Fig.  5. — Block  lava  on  Mauna  Loa 


in  this  way  proper  cuts  .are  formed  on  the  edges  of  the 
lava  streams,  passing  through  the  old  volcano  and  through 
the  sedimentary  strata  at  deeper  levels. 

The  old  volcano  of  Monte  Venda,  near  Padua,  affords 
an  interesting  example  of  this  type.  We  can  observe 
there  how  the  sedimentary  limestone  has  been  changed 
by  the  lava,  which  was  flowing  over  it,  into  marble  to  a 
depth  of  about  1  m.  (3  ft.)  Sometimes  the  limestone 
which  is  lying  over  the  lava  has  also  undergone  the  same 
transformation,  which  would  indicate  that  lava  has  not 
only  been  flowing  above  the  edge  of  the  crater,  but  has 

also  forced  itself  out  on  the  'sides  through  the  fissures 

10 


crs 


•<    CO 
P 
> 


WORLDS  IN   THE  MAKING 


between  two  layers  of  limestone.  Massive  subterranean 
lava  streams  of  this  kind  are  found  in  the  so-called  lak- 
kolithes  of  Utah  and  in  the  Caucasus.  There  the  supe- 
rior layers  have  been  forced  upward  by  the  lava  pressing 
from  below;  the  lava  froze,  however,  before  it  reached 
the  surface  of  the  earth,  where  it  might  have  formed  a 
volcano.  Quite-  a  number  of  granites,  the  so-called 
batholithes,  chiefly  occurring  in  Norway,  Scotland,  and 
Java,  are  of  similar  origin.  Occasionally  it  is  only  the 
core  of  congealed  lava  that  has  remained  of  the  whole 

volcano.  These  cores, 
which  originally  filled 
the  pipe  of  the  crater, 
are  frequent  in  Scot- 
land and  in  North 
America,  where  they 
are  designated  "necks" 
(Fig.  7). 

The  so-called  canons 
of  the  Colorado  Pla- 
teau, with  their  almost 
vertical  walls,  are  the 
results  of  the  erosive 

action  of  rivers.  A  drawing  by  Button  shows  a  wall  of 
this  kind  more  than  800  m.  (2600  ft.)  in  height,  through 
four  fissures  of  which  lava  streams  have  forced  their  way 
up  to  the  surface  (Fig.  8).  Over  one  of  these  fissures  a 
small  cone  of  volcanic  ashes  is  still  visible,  while  the  cones 
which  probably  overtopped  the  three  other  fissures  have 
been  washed  away,  so  that  the  veins  end  in  small  "necks." 
Evidently  a  very  fluid  lava — strong  percentages  of  mag- 
nesia and  of  oxide  of  iron  render  the  lava  more  fluid 
than  an  admixture  of  silicic  acid,  and  the  fluidity  is 

further  increased  by  the  presence  of  water — has  been 

12 


Fig.  7. — Mato    Tepee  in  Wyoming, 
U.  S.  A.     Typical  volcanic  "  Neck  " 


VOLCANIC    PHENOMENA   AND    EARTHQUAKES 

forced  into  the  fissures  which  were  already  present,  and 
has  reached  the  surface  of  the  earth  before  it  froze.    The 


t 


\ 


Fig.  8. — Clefts  filled  with  lava  and  volcanic  cone  of  ashes,  Torowheap 
Canon,  Plateau  of  Colorado.       Diagram. 

driving  force  behind  them  must  have  been  pretty  strong; 
else  the  lava  streams  could  not  have  attained  the  neces- 
sary velocity  of  flow. 

When  the  Krakatoa  was  blown  into  the  air  in  1883 
half  of  the  volcano  remained  behind.  This  half  clearly 
shows  the  section  of  the  cone  of  ashes,  which  has  been 
but  very  slightly  affected  by  the  destructive  action  of 
the  water.  We  find  there  in  the  central  part  the  light- 
colored  stopper  of  lava  in  the  volcano  pipe,  and  issuing 
from  it  more  light-colored  beds  of  lava,  between  which 
darker  strata  of  ashes  can  be  seen. 

The  distribution  of  volcanoes  over  the  surface  of  the 
earth  is  marked  by  striking  regularities.  Almost  all  the 
volcanoes  are  situated  near  the  shores  of  the  sea.  A  few 
are  found  in  the  interior  of  East  Africa;  but  they  are, 
at  any  rate,  near  the  Great  Lakes  of  the  equatorial 


WORLDS    IN    THE   MAKING 

regions.  The  few  volcanoes  which  are  supposed  to  be 
situated  in  Central  Asia  must  be  regarded  as  doubtful. 
We  miss,  however,  volcanoes  on  some  sea-coasts,  as  in 
Australia  and  along  the  long  coast-lines  of  the  Northern 
Arctic  Ocean  to  the  north  of  Asia,  Europe,  and  America. 
Volcanoes  occur  only  where  great  cracks  occur  in  the 
crust  of  the  earth  along  the  sea-coast.  Where  such  fis- 
sures are  found,  but  where  the  sea  or  large  inland  lake 
basins  are  not  near — as,  for  instance,  in  the  Austrian 
Alps — we  do  not  meet  with  any  volcanoes ;  such  districts 
are,  however,  renowned  for  their  earthquakes. 

Since  ancient  ages  the  belief  has  been  entertained  that 
the  molten  masses  of  the  interior  of  the  earth  find  an 
outlet  through  the  volcanoes..  Attempts  have  been  made 
to  estimate  the  depth  of  the  hearths  of  volcanoes,  but 
very  different  values  have  been  deduced.  Thus,  the 
hearth  under  the  volcano  of  Monte  Nuovo,  which  was 
thrown  up  in  the  year  1538  on  the  Phlegrsean  Fields,  near 
Naples,  has  been  credited  with  depths  varying  from  1.3  km. 
to  60  km.  (1  mile  to  40  miles) ;  for  the  Krakatoa,  estimates 
of  more  than  50  km.  (30  miles)  have  been  made.  All  these 
calculations  are  rather  aimless;  for  the  volcanoes  are 
probably  situated  on  folds  of  the  earth-crust,  through 
which  the  fluid  mass  (the  magma)  rushes  forth  in  wedges 
from  the  interior  of  the  earth,  and  it  will  presumably  be 
very  "difficult  to  say  where  the  hearth  of  magma  ends 
and  where  the  volcanic  pipe  commences.  The  Kilauea 
gives  the  visitor  the  impression  that  he  is  standing  over 
an  opening  in  the  crust  of  the  earth,  through  which  the 
molten  mass  rushes  forth  directly  from  the  interior  of  the 
earth.  (Fig.  9.) 

As  regards  the  earth-crust,  we  know  from  observations 
in  bore-holes  made  in  different  parts  of  the  world  that 
the  temperature  increases  rather  rapidly  with  the  depth, 

14 


VOLCANIC   PHENOMENA   AND    EARTHQUAKES 

on  an  average  by  about  thirty  degrees  Cent,  per  kilometre 
(about  1.6°  F.  per  100  feet).  It  must  be  remarked,  how- 
ever, that  the  depth  of  our  deepest  bore-holes  hardly 
exceeds  2  km.  (Paruchowitz,  in  Silesia,  2003  m.,  or 
6570  ft.;  Schladebach,  near  Merseburg,  Prussian  Saxony, 
1720  m.).  If  the  temperature  should  go  on  increasing 
at  the  rate  of  30  degrees  Cent,  for  each  further  kilometre, 
the  temperature  at  a  depth  of  40  kilometres  should  attain 
degrees  at  which  all  the  common  rocks  would  melt.  But 
the  melting-point  certainly  rises  at  the  same  time  as  the 
pressure.  The  importance  of  this  circumstance  was, 


Fig.  9. — The  Kilauea  Crater  on  Hawaii 

however,  much  exaggerated  when  it  was  believed  that  for 
this  reason  the  interior  of  the  earth  might  possibly  be 
solid.  Tammann  has  shown  by  direct  experiments  that 
the  temperature  of  fusion  only  rises  up  to  a  certain  press- 

15 


WORLDS    IN   THE   MAKING 

ure,  and  that  it  begins  to  decrease  again  on  a  further 
increase  of  pressure.  The  depths  indicated  above  are 
therefore  not  quite  correct.  If  we  assume,  however,  that 
other  kinds  of  rock  behave  like  diabase — the  melting- 
point  of  which,  according  to  the  determinations  of  Barus, 
rises  by  1°  Cent,  for  each  40  atmospheres  of  pressure 
corresponding  to  a  depth  of  155  m. — we  should  conclude 
that  the  solid  crust  of  the  earth  could  not  have  a  greater 
thickness  than  50  or  60  km.  (40  miles).  At  greater 
depths  we  should  therefore  penetrate  into  the  fused 
mass.  On  account  of  its  smaller  density  the  silicic  acid 
will  be  concentrated  in  the  upper  strata  of  the  molten 
mass,  while  the  basic  portions  of  the  magma,  which  are 
richer  in  iron  oxide,  will  collect  in  the  lower  strata,  owing 
to  their  greater  density. 

This  magma  we  have  to  picture  to  ourselves  as  an  ex- 
tremely viscid  liquid  resembling  asphalt.  The  experi- 
ments of  Day  and  Allen  show  that  rods,  supported  at 
their  ends,  of  30  x  2  x  1  mm.  of  different  minerals,  like  the 
feldspars  microcline  and  albite,  could  retain  their  shape  for 
three  hours  without  curving  noticeably,  although  their 
temperature  was  about  a  hundred  degrees  above  their 
melting-point,  and  although  they  appeared  completely 
fused,  or,  more  correctly,  completely  vitrified-^wihen  taken 
out  of  the  furnace.  These  molten  silicates  behave  very 
differently  from  other  liquids  like  water  and  mercury,  with 
which  we  are  more  accustomed  to  deal. 

The  motion  and  diffusion  in  the  magma,  and  especially 
in  the  very  viscous  and  sluggish  acid  portions  of  the  upper 
strata,  will  therefore  be  exceedingly  small,  and  the  mag- 
ma will  behave  almost  like  a  solid  body,  like  the  minerals 
of  the  experiments  of  Day  and  Allen.  The  magmas  of 
volcanoes  like  Etna,  Vesuvius,  and  Pantellaria  may, 
therefore,  have  quite  different  compositions,  as  we  should 

16 


VOLCANIC    PHENOMENA   AND    EARTHQUAKES 

conclude  from  their  lavas  without  our  being  forced  to 
believe,  with  Stubel,  that  these  three  hearths  of  vol- 
canoes are  completely  separated,  though  not  far  removed 
from  one  another.  In  the  lava  of  Vesuvius  a  tempera- 
ture of  1000  or  1100  degrees  has  been  found  at  the  lower 
extremity  of  the  stream.  From  the  occurrence  in  the 
lava  of  certain  crystals  like  leucite  and  olivin,  which  we 
have  reason  to  assume  must  have  been  formed  before 
the  lava  left  the  crater,  it  has  been  concluded  that  the 
lava  temperature  cannot  have  been  higher  than  1400 
degrees  before  it  left  the  volcanic  pipe. 

It  would,  however,  be  erroneous  to  deduce  from  the 
temperature  of  the  lava  of  Vesuvius  that  the  hearth  of 
the  volcano  must  be  situated  at  a  depth  of  approximate- 
ly 50  kilometres.  Most  likely  its  depth  is  much  smaller, 
perhaps  not  even  10  kilometres.  For  there,  as  every- 
where where  volcanoes  occur,  the  crust  of  the  earth  is 
strongly  furrowed/and  the  magma  will  just  at  the  spots 
where  we  find  volcanoes  come  much  nearer  to  the  sur- 
face of  the  earth  than  elsewhere. 

The  importance  of  water  for  the  formation  of  vol- 
canoes probably  lies  in  the  fact  that,  in  the  neighbor- 
hood of  cracks  under  the  bottom  of  the  sea,  the  water 
penetrates  down  to  considerable  depths.  When  the 
water  reaches  a  stratum  of  a  temperature  of  365  degrees 
--the  so-called  critical  temperature  of  water — it  can  no 
longer  remain  in  the  liquid  state.  That  would  not  prevent, 
however,  its  penetrating  still  farther  into  the  depths,  in 
spite  of  its  gaseous  condition.  As  soon  as  the  vapor  comes 
in  contact  with  magma,  it  will  eagerly  be  absorbed  by  the 
magma.  The  reason  is  that  water  of  a  temperature  of 
more  than  300  degrees  is  a  stronger  acid  than  silicic  acid ; 
the  latter  is  therefore  expelled  by  it  from  its  compounds, 
the  silicates,  which  form  the  main  constituents  of  the 

17 


WORLDS    IN   THE    MAKING 

magma.  The  higher  the  temperature,  the  greater  the 
power  of  the  magma  to  absorb  water.  Owing  to  this 
absorption  the  magma  swells  and  becomes  at  the  same 
time  more  fluid.  The  magma  is  therefore  pressed  out  by 
the  action  of  a  pressure  which  is  analogous  to  the  osmotic 
pressure  by  virtue  of  which  water  penetrates  through  a 
membrane  into  a  solution  of  sugar  or  salt.  This  press- 
ure may  become  equivalent  to  thousands  of  atmos- 
pheres, and  this  very  pressure  would  raise  the  magma 
up  the  volcanic  pipe  even  to  a  height  of  6000  in. 
(20,000  feet)  above  the  sea-level.  As  the  magma  is  as- 
cending in  the  volcanic  pipe  it  is  slowly  cooled,  and  its 
capacity  for  binding  water  diminishes  with  falling  tem- 
perature. The  water  will  hence  escape  under  violent 
ebullition,  tearing  drops  and  larger  lumps  of  lava  with 
it,  which  fall  down  again  as  ashes  or  pumice-stone.  After 
the  lava  has  flown  out  of  the  crater  and  is  slowly  cooling, 
it  continues  to  give  off  water,  breaking  up  under  the  for- 
mation of  block  lava  (see  Fig.  5).  If,  on  the  other  hand, 
the  lava  in  the  crater  of  the  volcano  is  comparatively  at 
rest,  as  in  Kilauea,  the  water  will  escape  more  slowly; 
owing  to  the  long  -  continued  contact  of  the  surface 
layer  of  lava  with  the  air,  little  water  will  remain  in  it, 
the  water  being,  so  to  say,  removed  by  aeration,  and  the 
lava  streams  will  therefore,  when  congealing,  form  more 
smooth  surfaces. 

In  some  cases  volcanoes  have  been  proved  (Stiibel  and 
Branco)  not  to  be  in  connection  with  any  fractures  in 
the  crust  of  the  earth.  That  holds,  for  instance,  for  sev- 
eral volcanoes  of  the  early  Tertiary  age  in  Swabia.  We 
may  imagine  that  the  pressure  produced  by  the  swelling 
of  the  magma  became  so  powerful  as  to  be  able  to  break 
through  the  earth-crust  at  thinner  spots,  even  in  the 
absence  of  previous  fissures. 

18 


VOLCANIC   PHENOMENA   AND    EARTHQUAKES 

If,  in  our  consideration,  we  follow  the  magma  farther 
into  the  depths,  we  shall  not  find  any  reason  for  assum- 
ing that  the  temperature  will  not  rise  farther  towards  the 
interior  of  the  earth.  At  depths  of  300  or  400  km.  (250 
miles)  the  temperature  must  finally  attain  degrees  such 
that  no  substance  will  be  able  to  exist  in  any  other  state 
than  the  gaseous.  Within  this  layer  the  interior  of  the 
earth  must,  therefore,  be  gaseous.  From  our  knowledge 
of  the  behavior  of  gases  at  high  temperatures  and  press- 
ures, we  may  safely  conclude  that  the  gases  in  the  cen- 
tral portions  of  the  earth  will  behave  almost  like  an  ex- 
tremely viscid  magma.  In  certain  respects  they  may 
probably  be  compared  to  solid  bodies;  their  compressi- 
bility, in  particular,  will  be  very  small. 

We  might  think  that  we  could  not  possibly  learn  any- 
thing concerning  the  condition  of  those  strata.  Earth- 
quakes have,  however,  supplied  us  with  a  little  informa- 
tion. Such  gaseous  masses  must  fill  by  far  the  greatest 
part  of  the  earth,  and  they  must  have  a  very  high  specific 
gravity;  for  the  average  density  of  the  earth  is  5.52,  and 
the  outer  strata,  the  ocean  and  the  masses  of  the  surface 
which  are  known  to  us,  have  smaller  densities.  The  or- 
dinary rocks  possess  a  density  ranging  from  2.5  to  3. 
It  must,  therefore,  be  assumed  that  the  materials  of  the 
innermost  portions  of  the  earth  must  be  metallic,  and 
Wiechert,  in  particular,  has  advocated  this  view.  Iron 
will  presumably  form  the  chief  constituent  of  this  gas 
of  the  central  earth.  Spectrum  analysis  teaches  us  that 
iron  is  a  very  important  constituent  of  the  sun.  We 
know,  further,  that  the  metallic  portions  of  the  meteo- 
rites consist  essentially  of  iron;  and  finally  terrestrial 
magnetism  indicates  that  there  must  be  large  masses  of 
iron  in  the  interior  of  the  earth.  We  have  also  reason 
to  believe  that  the  native  iron  occurring  in  nature — e.  g., 

19 


WORLDS    IN    THE    MAKING 

the  well-known  iron  of  Ovifak,  in  Greenland — is  of  vol- 
canic origin.  The  materials  in  the  gaseous  interior  of 
the  earth  will,  owing  to  their  high  density,  behave  in 
chemical  and  physical  respects  like  liquids.  As  sub- 
stances like  iron  will,  also  at  very  high  temperatures, 
have  a  far  higher  specific  gravity  than  their  oxides,  and 
these  again  have  a  higher  gravity  than  their  silicates, 
we  have  to  assume  that  the  gases  in  the  core  of  the  earth 
will  almost  exclusively  be  metallic,  that  the  outer  por- 
tions of  the  core  will  contain  essentially  oxides,  and  those 
farther  out  again  mostly  silicates. 

The  fused  magma  will,  on  penetrating  in  the  shape  of 
batholithes  into  the  upper  layers,  probably  be  divided 
into  two  portions,  of  which  one,  the  lighter  and  gaseous, 
will  contain  water  and  substances  soluble  in  it;  while 
the  other,  heavier  portion,  will  essentially  consist  of  sili- 
cates with  a  lower  percentage  of  water.  The  more  fluid 
portion,  richer  in  water,  will  be  secreted  in  the  higher 
layers,  will  penetrate  into  the  surrounding  sedimentary 
strata,  especially  into  their  fissures,  and  will  fill  them 
with  large  crystals,  often  of  metallurgical  value — e.g.,  of 
the  ores  of  tin,  copper,  and  other  metals,  while  the  water 
will  slowly  evaporate  through  the  superposed  strata.  The 
more  viscid  and  sluggish  mass  of  silicates,  on  the  other 
hand,  will  congeal,  thanks  to  its  great  viscosity,  to  glass, 
or,  when  the  cooling  is  very  slow,  to  small  crystals. 

We  now  turn  to  earthquakes.  No  country  has  been 
absolutely  spared  by  earthquakes.  In  the  districts  bound- 
ing upon  the  Baltic,  and  especially  in  northern  Russia, 
they  have,  however,  been  of  a  quite  harmless  type.  The 
reason  is  that  the  earth-crust  there  has  been  lying  un- 
disturbed for  long  geological  epochs  and  has  never  been 
fractured.  The  comparatively  severe  earthquake  which 

shook  the  west  coast  of  Sweden  on  October  23,  1904,  to 

20 


VOLCANIC    PHENOMENA   AND    EARTHQUAKES 

an  unusually  heavy  degree,  without,  however,  causing 
any  noteworthy  damage  (a  few  chimneys  were  knocked 
over),  was  caused  by  a  fault  of  relatively  pronounced 
character  for  those  districts  in  the  Skager-Rack — a  con- 
tinuation of  the  deepest  fold  in  the  bottom  of  the  North 
Sea,  the  so-called  Norwegian  Trough,  which  runs  parallel 
to  the  Norwegian  coast.  In  Germany,  the  Vogtland  and 
the  districts  on  both  sides  of  the  middle  Rhine  have  fre- 
quently been  visited  by  earthquakes.  Of  other  European 
countries,  Switzerland,  Spain,  Italy,  and  the  Balkan  Pen- 
insula, as  well  as  the  Karst  districts  of  Austria,  have 
often  suffered  from  earthquakes. 

According  to  the  committee  appointed  by  the  British 
Association  for  the  investigation  of  earthquakes — a  com- 
mittee which  has  contributed  a  great  deal  to  our  knowl- 
edge of  these  great  natural  phenomena — earthquakes  of 
some  importance  emanate  from  certain  centres  which 
have  been  indicated  on  the  subjoined  map  (Fig.  10).  The 
most  important  among  these  regions  comprises  Farther 
India,  the  Sunda  Isles,  New  Guinea,  and  Northern  Aus- 
tralia; it  is  marked  on  the  map  by  the  letter  F.  From 
this  district  have  emanated  in  the  six-year  period  1899- 
1904  no  fewer  than  249  earthquakes,  which  have  been 
recorded  in  many  observatories  far  removed  from  one 
another.  This  earthquake  centre  F  is  closely  related  to 
the  one  marked  E,  in  Japan,  from  which  189  earthquakes 
have  proceeded.  Next  to  this  comes  the  extensive  dis- 
trict K  with  174  earthquakes,  comprising  the  most  im- 
portant folds  in  the  crust  of  the  Old  World,  including  the 
mountain  chains  from  the  Alps  to  the  Himalaya.  This 
district  is  interesting,  because  it  has  been  disturbed  by 
a  great  many  earthquakes,  although  it  is  almost  entirely 
situated  on  the  Continent.  After  that  we  have  the  dis- 
districts  A,  B,  C,  with  125,  98,  and  95  earthquakes.  They 

21 


WORLDS    IN   THE    MAKING 

are  situated  near  lines  of  fracture  in  the  earth -crust 
along  the  American  coast  of  the  Pacific  Ocean  and  the 
Caribbean  Sea.  District  D,  with  78  earthquakes,  is  simi- 
larly situated.  The  three  last  -  mentioned  districts,  B, 
C,  D,  as  well  as  G,  between  Madagascar  and  India,  with 
85  earthquakes,  all  seem  to  be  surpassed  by  the  district 
H  in  the  eastern  Atlantic,  with  its  107  earthquakes. 
These  latter  are,  however,  relatively  feeble,  and  we  owe 
their  accurate  records  probably  to  the  circumstances  that 
a  great  many  earthquake  observatories  are  situated  with- 
in the  immediate  surroundings  of  this  district.  The  same 
may  be  said  of  the  district  I,  or  Newfoundland,  which  is 
not  characterized  by  many  earthquakes,  and  of  the  dis- 
trict J,  between  Iceland  and  Spitzbergen,  with  31  and  19 


Fig.  10. — Chief  earthquake  centres,  according  to  the  British 
Association  Committee 

earthquakes  respectively.    The  last  on  the  list  used  to 
be  the  district  L,  situated  about  the  South  Pole,  with  only 

eight  earthquakes.    This  small  number  is  probably  inere- 

22 


VOLCANIC   PHENOMENA  AND   EARTHQUAKES 

ly  due  to  the  want  of  observatories  in  those  parts  of  the 
earth.  Another  district,  M,  has  finally  been  added, 
which  extends  to  the  southwest  from,New  Zealand.  No 
fewer  than  75  intense  earthquakes  were  recorded  between 
March  14  and  November  23, 1903,  by  the  Discovery  Expe- 
dition,^ 70°  sou  them  latitude  and  178°  eastern  longitude. 

Earthquakes  commonly  occur  in  swarms  or  groups. 
Thus,  more  than  2000  shocks  were  counted  on  Hawaii 
in  March,  1868.  During  the  earthquakes  which  de- 
vastated the  district  of  Phokis,  in  Greece,  in  1870-73, 
shocks  succeeded  one  another  for  a  long  time  at  intervals 
of  three  seconds.  During  the  whole  period  of  three  and 
a  half  years  about  half  a  million  shocks  were  counted, 
and,  further,  a  quarter  of  a  million  subterranean  reports 
which  were  not  accompanied  by  noticeable  concussions. 
Yet  of  all  these  shocks  only  about  300  did  noteworthy 
damage,  and  only  35  were  considered  worth  being  re- 
ported in  the  newspapers.  The  concussion  of  October 
23,  1904,  belonged  to  a  group  which  lasted  from  October 
10  to  October  28,  and  in  which  numerous  small  tremors 
were  noticed,  especially  on  October  24  and  25.  The 
earthquake  of  San  Francisco  commenced  on  April  18, 
1906,  at  5  hrs.  12  min.  6  sec.  A.M.  (Pacific  Ocean  time), 
and  ended  at  5  hrs.  13  min.  11  sec.,  lasting  therefore 
1  minute  and  5  seconds.  Twelve  smaller  shocks  s,ucceed- 
ed  in  the  following  hour.  Before  6  hrs.  52  min.  P.M., 
nineteen  further  concussions  were  counted,  and  various 
smaller  shocks  succeeded  in  the  following  days. 

With  such  groups  of  earthquakes  weaker  tremors  usually 
precede  the  violent  destructive  shocks  and  give  a  warn- 
ing. Unfortunately  this  is  not  always  so,  and  no  warning 
was  given  by  the  earthquakes  which  destroyed  Lisbon  in 
1755  and  Caracas  in  1812,  nor  by  those  which  devastated 
Agram  in  1880,  nor,  finally,  in  the  case  of  the  San  Fran- 
3  23 


WORLDS    IN   THE   MAKING 

cisco  disaster.  A  not  very  severe  earthquake  without 
feebler  precursors  befell  Ischia  in  1881,  while  the  violent 
catastrophe  which  devastated  this  magnificent  island  in 
1883  was  heralded  by  several  warnings.  As  in  San  Fran- 
cisco and  Chili  in  1906,  less  violent  concussions  generally 
succeed  the  destructive  shocks.  Earthquakes  like  that 
of  Lisbon  in  1755,  consisting  of  a  single  shock,  are  very 
rare.  * 

The  violent  concussions  often  produce  large  fissures 
in  the  ground.  Such  were  noticed  in  several  places  at 
San  Francisco.  One  of  the  largest  fissures  known,  that  of 
Midori,  in  Japan,  was  caused  by  the  earthquake  of  Oc- 
tober 20,  1891.  It  left  a  displacement  of  the  ground 
ranging  up  to  6  in.  (20  ft.)  in  the  vertical  and  4  m. 
(13  ft.)  in  the  horizontal  direction.  This  crack  had  a 
length  of  not  less  than  65  km.  (40  miles).  Extensive 
fissures  were  also  formed  by  the  earthquakes  of  Calabria, 
in  1783,  at  Monte  San  Angelo,  and  in  the  sandstones  of  the 
Balpakram  Plateau  in  India,  in  1897.  In  mountainous 
districts  falls  of  rock  are  a  frequent  consequence  of  the 
formation  of  fissures  and  earthquakes.  A  large  number 
of  rocks  fell  in  the  neighborhood  of  Delphi  during  the 
Phokian  earthquake.  On  January  25,  1348,  an  earth- 
quake sent  down  a  large  portion  of  Mount  Dobratsch  (in 
the  Alps  of  Villach,  in  Carinthia,  which  is  now  much  fre- 
quented by  tourists)  and  buried  two  towns  and  seventeen 
villages.  The  earthquake  of  April  18,  1906,  in  California 
started  from  a  crack  which  extends  from  the  mouth  of 
Alder  Creek,  near  Point  Arena,  running  parallel  with  the 
coast-line  mostly  inland,  then  entering  the  sea  near  San 
Francisco,  and  turning  again  inland  between  Santa  Cruz 
and  San  Jose,  finally  proceeding  via  Chittenden  up  to 
Mount  Pinos,  a  distance  of  about  600  km.  (400  miles), 
in  the  direction  of  N.  35°  W.  to  S.  35°  E.  Along  this 

24 


VOLCANIC    PHENOMENA   AND    EARTHQUAKES 


Fig.  11.— Clefts  in  Valentia  Street,  San  Francisco,  after  the 
earthquake  of  1906 

crack  the  two  masses  of  the  earth  have  been  displaced  so 
that  the  ground  situated  to  the  southwest  of  the  fissure 
has  been  moved  by  about  3  m.  (10  ft.),  and  in  some 
spots  even  by  6  m.  (20  ft.)  towards  the  northwest.  In 
some  localities  in  Sonoma  and  Mendocino  counties  the 
southwestern  part  has  been  raised,  but  nowhere  by  more 
than  1.2  m.  (4  ft.).  This  is  the  longest  crack  which  has 
ever  been  noticed  in  connection  with  an  earthquake. 

The  earthquake  over,  the  ground  does  not  always  return 
to  its  original  position,  but  remains  in  a  more  or  less  wavy 
condition.  This  can  most  easily  be  observed  in  districts 
where  streets  or  railways  cross  the  ground.  It  is  report- 
ed, for  instance,  that  the  track  of  the  tramway-lines  in 
Market  Street,  the  chief  thoroughfare  of  San  Francisco, 
formed  large  wavelike  curves  after  the  earthquake. 

As  a  consequence  of  the  displacements  in  the  interior  of 
the  earth  and  of  the  formation  of  fissures,  river  courses  are 
changed,  springs  become  exhausted,  and  new  springs  arise. 
That  was  the  case,  for  instance,  in  California  in  1906.  The 

25 


WORLDS   IN   THE   MAKING 

ground  water  often  rushes  out  with  considerable  violence, 
tearing  with  it  sand  and  mud  and  stones,  and  piling  them 
up,  occasionally  forming  little  craters  (Fig.  12).  Extensive 
floods  may  also  be  caused  on  such  occasions.  By  such  a 
flood  the  ancient  Olympia  was  submerged  under  a  layer 
of  river  sand  which  for  some  time  preserved  from  destruc- 
tion the  ancient  Greek  masterpieces  of  art — among  them 
the  famous  statue  of  Hermes.  The  floods  afterwards  re- 
ceded, and  the  treasures  of  ancient  Olympia  CuulJ  be 
excavated. 

Like  the  natural  water  channels  and  arteries  in  the  in- 
terior of  the  earth,  water  mains  are  displaced  by  the  con- 
cussions. The  direct  damage  caused  by  the  floods  is  often 
less  important  than  the  damage  due  to  the  impossibility 
of  extinguishing  the  fires  which  follow  the  destruction  of 
the  buildings.  It  was  the  fires  that  did  most  of  the  enor- 
mous material  damage  in  the  destruction  of  San  Fran- 
cisco. 

Still  greater  devastation  is  wrought  by  the  ocean  waves 
thrown  up  by  earthquakes.  We  have  already  referred  to 
the  flood  of  Lisbon  in  1755,  which  was  felt  on  the  western 
coast  of  Norway  and  Sweden.  Another  wave,  in  1510,  de- 
voured 109  mosques  and  1070  houses  in  Constantinople. 
Another  wave,  again,  invaded  Kamaishi,  in  Japan,  on  June 
15, 1896,  swept  away  7600  houses  and  killed  27,000  people. 

We  have  repeatedly  alluded  to  the  disastrous  flood- 
wave  of  Krakatoa  of  1883.  This  wave  traversed  the 
whole  of  the  Indian  Ocean,  passing  to  the  Cape  of  Good 
Hope  and  Cape  Horn,  and  travelled  round  half  the  globe 
afterwards.  Even  more  remarkable  was  the  aerial  wave, 
which  spread  like  an  explosion  wave. 

While  the  most  violent  cannonades  are  rarely  heard 
for  more  than  150  km.  (95  miles) — in  a  single  case 
at  a  distance  of  270  km.  (170  miles) — the  eruption  of 

26 


VOLCANIC   PHENOMENA   AND    EARTHQUAKES 

Krakatoa  was  heard  at  Alice  Springs,  at  a  distance  of 
3600  kilometres,  and  on  the  island  of  Rodriguez,  at 
almost  4800  km.  (3000  miles).  The  barographs  of  the 
meteorological  stations  first  marked  a  sudden  rise  and 
then  a  decided  sinking  of  the  air  pressure,  succeeded  by  a 
few  smaller  fluctuations.  These  air  pulses  were  repeated 
in  some  places  as  many  as  seven  times.  We  may  therefore 
assume  that  the  aerial  wave  passed  these  places  three 


Fig.  12. — Sand  craters  and  fissures,  produced  by  the  Corinth  earth- 
quake of  1861.     In  the  water,  branches  of  flooded  trees 

times  in  the  one  direction,  and  three  times  in  the  other, 
travelling  round  the  earth.  The  velocity  of  propagation 
of  this  wave  was  314.2  m.  (1030  ft.)  per  second,  corre- 
sponding to  a  temperature  of — 27°  Cent.  (17°  F.)  which 
prevails  at  an  altitude  of  about  8  km.  (5  miles)  above  the 

27 


WORLDS    IN   THE   MAKING 

earth's  surface,  at  which  altitude  this  wave  may  have 
travelled. 

Within  the  last  decade  a  peculiar  phenomenon  (leading 
to  what  is  designated  variation  of  latitudes)  has  been 
studied.  The  poles  of  the  axis  of  the  earth  appear  to 
move  in  a  very  irregular  curve  about  their  mean  axis. 
The  movement  is  exceedingly  small.  The  deviation  of  the 
North  Pole  from  its  mean  position  does  not  amount  to  more 
than  10  m.  (about  33  ft.).  It  has  been  believed  that  these 
motions  of  the  North  Pole  are  subject  to  sudden  fluctua- 
tions after  unusually  violent  earthquakes,  especially  when 
such  concussions  follow  at  rapid  intervals.  That  would 
give  us,  perhaps  more  than  any  other  observation,  an 
idea  of  the  force  of  earthquakes,  since  they  would  appear 
to  be  able  to  disturb  the  equilibrium  of  the  whole  mass 
of  our  globe. 

A  severely  felt  effect  of  earthquakes,  though  most  peo- 
ple perhaps  pay  little  attention  to  it,  is  the  destruction  of 
submarine  cables.  The  gutta-percha  sheaths  of  cables  are 
frequently  found  in  a  fused  condition,  suggesting  volcanic 
eruptions  under  the  bottom  of  the  sea.  We  take  care  now 
to  avoid  earthquake  centres  in  laying  telegraphic  cables. 
Their  positions  have  been  ascertained  by  the  most  modern 
investigations  (see  Fig.  10). 

People  have  always  been  inclined  to  look  for  a  connec- 
tion between  earthquakes  and  volcanic  eruptions.  The 
connection  is  unquestionable  in  a  large  number  of  violent 
earthquakes.  In  order  to  establish  it,  the  above-men- 
tioned committee  of  the  British  Association  has  compiled 
the  following  table  of  the  history  of  the  earthquakes  of 
the  Antilles: 


1692. — Port   Royal,  Jamaica,  destroyed  by   an   earthquake; 
land  sinking  into  the  sea.     Eruption  on  St.  Kitts. 

28 


VOLCANIC   PHENOMENA   AND   EARTHQUAKES 

1718. — Terrible  earthquake  on  St.  Vincent,  followed  by  an 
eruption. 

1766-67. — Great  shocks  in  northeastern  South  America,  in 
Cuba,  Jamaica,  and  the  Antilles.  Eruption  on  Santa  Lucia. 

1797. — Earthquake  in  Quito,  loss  of  40,000  lives.  Concus- 
sions in  the  Antilles,  eruption  on  Guadeloupe. 

1802. — Violent  shocks  in  Antigua.     Eruption  on  Guadeloupe. 

1812. — Caracas,  capital  of  Venezuela,  totally  destroyed  by 
earthquake.  Violent  shocks  in  the  Southern  States  of  North 
America,  commencing  on  November  11,  1811.  Eruptions  on  St. 
Vincent  and  Guadeloupe. 

1835-36. — Violent  concussions  in  Chili  and  Central  America. 
Eruption  on  Guadeloupe. 

1902. — April  19.  Violent  shocks,  destroying  many  towns  of 
Central  America.  Mont  Pelee,  on  Martinique,  in  activity. 
Eruption  on  May  3.  Submarine  cables  break,  sea  recedes. 
Renewed  violent  movements  of  the  sea  on  May  8,  19,  20.  Erup- 
tion on  St.  Vincent,  cable  destroyed  on  May  7.  Violent  erup- 
tion of  Mont  Pelee  on  May  8.  Destruction  of  St.  Pierre. 
Numerous  smaller  earthquakes. 

This  table  distinctly  marks  the  restless  state  of  affairs 
in  that  part  of  the  earth,  and  how  quiet  and  safe  matters 
are  comparatively  in  old  Europe,  especially  in  the  north. 
Some  parts  of  Central  America  are  so  persistently  visited 
by  earthquakes  that  one  of  them,  Salvador,  has  been 
christened  "  Schaukelmatte."  It  is  not  saying  too  much 
to  assert  that  the  earth  is  there  incessantly  trembling. 
Other  districts  which  are  very  frequently  visited  are  the 
Kuriles  and  Japan,  as  well  as  the  East  Indian  'islands. 
In  all  these  countries  the  crust  of  the  earth  has  been  broken 
and  folded  within  comparatively*  recent  epochs  (chiefly  in 
the  Tertiary  age)  by  numerous  fissures,  and  their  com- 
pression is  still  going  on. 

The  smaller  earthquakes,  of  which  not  less  than  30,000 
are  counted  in  the  course  of  a  year,  do  not  stand  in  any 
closer  relation  to  volcanic  eruptions.  This  is  also  the 

29 


Fig.  13. — Earthquake  lines  in  lower  Austria 


VOLCANIC   PHENOMENA   AND    EARTHQUAKES 

case  for  a  number  of  large  earthquakes,  among  which  we 
have  to  count  the  San  Francisco  earthquake. 

It  is  assured  with  good  reason  that  earthquakes  are 
often  produced  at  the  bottom  of  the  sea,  where  there  is  a 
strong  slope,  by  slips  of  sedimentary  strata  which  have 
been  washed  down  from  the  land  into  the  sea  in  the  course 
of  centuries.  Milne  believes  that  the  seaquake  of  Kamai- 
shi1  of  June  15, 1896,  was  of  this  character.  Concussions 
may  even  be  promoted  by  the  different  loading  of  the 
earth  resulting  from  the  fluctuations  in  the  pressure  of 
the  air  above  it. 

Smaller,  though  occasionally  rather  violent,  earthquakes 
are  not  infrequent  in  the  neighborhood  of  Vienna.  On  the 
map  (Fig.  13)  we  see  three  lines.  The  line  A  B  is  called 
4the  thermal  line,  because  along  it  a  number  of  hot  springs, 
the  thermaB  of  Meidling,  Baden,  Voslau,  etc.,  are  located, 
which  are  highly  valued ;  the  other  line  B  C  is  called  the 
Kamp  line,  because  it  is  traversed  by  the  river  Kamp; 
and  the  third  B  F  is  called  the  Miirz  line,  after  the  river 
Miirz.  The  main  railway- track  between  Vienna  and  Bruck 
follows  the  valleys  of  A  B  and  E  F. 

These  lines,  which  probably  correspond  to  large  fissures 
in  the  earth-crust,  are  known  as  sources  of  numerous 
earthquakes.  The  district  about  Wiener  Neustadt,  where 
the  three  lines'  intersect,  is  often  shaken  by  violent  earth- 
quakes ;  some  of  their  dates  have  been  marked  on  the  map. 

The  curve  which  is  indicated  by  the  letters  X  X  on  the 
map  marks  the  outlines  of  an  earthquake  which  started 
on  January  3,  1873,  from  both  sides  of  the  Kamp  line. 
It  is  striking  to  see  how  the  earthquake  spread  in  the 
loose  ground  of  the  plain  between  St.  Polten  and  Tulln, 
while  the  masses  of  rock  situated  to  the  northwest  and 
southeast  formed  obstacles  to  the  propagation  of  the 
earthquake  waves. 

31 


WORLDS    IN    THE   MAKING 

Similar  conclusions  have  been  deduced  from  the  study 
of  the  spreading  of  the  waves  which  destroyed  Charles- 
ton, South  Carolina,  in  1886.  Twenty-seven  lives  were 
destroyed  by  this  shock.  It  was  the  most  terrible 
earthquake  that  ever  visited  the  United  States  be- 
fore the  year  1906.  In  the  Charleston  concussion  the 
Alleghany  Mountains  proved  a  powerful  bar  against  the 


g  14. — Library  building  of  Leland  Stanford  Junior  University,  in 
California,  after  the  earthquake  of  1906.  The  photograph  shows  the 
great  strength  of  iron  structures  in  comparison  to  the  strength  of 
brickwork.  The  effect  of  the  earthquake  on  wooden  structures 
can  be  seen  in  Fig.  11 

32 


VOLCANIC    PHENOMENA   AND    EARTHQUAKES 

further  propagation  of  the  shocks,  which  all  the  more 
easily  travelled  in  the  loose  soil  of  the  Mississippi  Valley. 
In  San  Francisco,  likewise,  the  worst  devastation  fell  upon 
those  parts  of  the  town  which  had  been  built  upon  the 
loose,  partly  made  ground  in  the  neighborhood  of  the 
harbor,  while  the  buildings  erected  on  the  famous  moun- 
tain ridges  of  San  Francisco  suffered  comparatively  little 
damage,  in  so  far  as  they  were  not  reached  by  the  destruc- 
tive fires.  As  regards  the  destructive  effects  of  the  earth- 
quake in  San  Francisco,  the  building-ground  of  that  city 
has  been  divided  into  four  classes  (the  first  is  the  safest, 
the  last  the  most  unsafe) — namely :  1 .  Rocky  soil.  2.  Valleys 
situated  between  rocks  and  filled  up  by  nature  in  the  course 
of  time.  3.  Sand-dunes.  4.  Soil  created  by  artificial  filling  up. 
This  latter  soil  "behaved  like  a  semiliquid  jelly  in  a  dish," 
according  to  the  report  of  the  Earthquake  Commission. 

For  similar  reasons  the  sky-scrapers,  constructed  of 
steel  on  deep  foundations,  stood  firmest.  After  them 
came  brick  houses,  with  well-joined  and  cemented  walls 
on  deep  foundations.  The  weakness  of  wooden  houses 
proved  mainly  due  to  the  poor  connection  of  the  beams, 
a  defect  which  might  easily  be  remedied.  The  superiority 
of  the  steel  structure  will  be  apparent  from  the  illustra- 
tions (Figs.  11  and  14). 

The  spots  situated  just  over  the  crack,  of  which  we 
spoke  on  page  25,  suffered  the  most  serious  damage.  Next 
to  them,  devastation  befell  especially  localities  which,  like 
Santa  Rosa,  San  Jose,  and  Palo  Alto  with  Leland  Stan- 
ford Junior  University,  are  situated  on  the  loose  soil  of  the 
valley,  whose  deepest  portions  are  covered  by  the  bay  of 
San  Francisco.  The  splendidly  endowed  California  Uni- 
versity, in  Berkeley,  and  the  famous  Lick  Observatory, 
both  erected  on  rocky  ground,  fortunately  escaped  with- 
out any  notable  damage. 

33 


WORLDS   IN   THE   MAKING 

The  map  sketch  (Fig.  15)  by  Suess  represents  the  earth- 
quake lines  of  Sicily  and  Calabria.  These  districts  have, 
as  mentioned  before,  been  devastated  by  severe  earth- 
quakes, of  which  the  most  terrible  occurred  in  the  year 
1783,  and  again  in  1905  and  1907.  They  have  also  been 
the  scene  of  many  smaller  concussions. 

The  bottom  of  the  Tyrrhenian  Sea  —  between  Italy, 
Sicily,  and  Sardinia — has  been  lowered  in  rather  recent  ages 


Fig.  15. — Earthquake  lines  in  the  Tyrrhenian  depression 

and  is  still  sinking.  We  notice  on  the  map  five  dotted  linos, 
corresponding  to  cracks  in  the  crust  of  the  earth.  These 
lines  would  intersect  in  the  volcanic  district  of  the  Lipari 
Islands.  We  further  see  a  dotted  circular  arc  correspond- 
ing to  a  fissure  which  is  regarded  as  the  source  of  the  Cala- 

brian  earthquakes  of  1783,  1905,  and  1907.    The  earth- 

34 


VOLCANIC  PHENOMENA  AND  EAKTHQUAKES 

crust  behaved  somewhat  after  the  manner  of  a  window- 
pane  which  was  burst  by  a  heavy  impact  from  a  point 
corresponding  to  the  Island  of  Lipari.  From  this  point 
radiate  lines  of  fracture,  and  fragments  have  been  broken 
off  from  the  earth-crust  by  arc-shaped  cracks,  The  vol- 
cano Etna  is  situated  on  the  intersection  of  the  radial  and 
circular  fissures. 

In  recognition  of  the  high  practical  importance  of  earth- 
quake observations,  seismological  stations  have  in  recent 
days  been  erected  in  many  localities.  At  these  observatories 
the  earthquakes  are  recorded  by  pendulums  whose  styles 
draw  lines  on  tapes  of  paper  moved  by  clock-work.  As  long 
as  the  earth  is  quiet  the  drawn  line  is  straight.  When  earth- 
quakes set  in,  the  line  passes  into  a  wavy  curve.  As  long  as 
the  movement  of  the  paper  is  slow,  the  curve  merely  looks 
like  a  widened  straight  line.  The  subjoined  illustration 
(Fig.  16)  represents  a  seismogram  taken  at  the  station  of 
Shide,  on  the  Isle  of  Wight,  on  August  31,  1898.  The 
earthquake  recorded  originated  in  the  Centre  G,  in  the 
Indian.  Ocean.  The  origin  has  been  deduced  from  the 


20.  36.  25. 
SO.  31.  21.  |  20.  42. 


Fig.  16. — Seismogram   recorded   at   Shide,  Isle   of   Wight,  on 
August  31,  1898 


moments  of  arrival  of  the  different  waves  at  different 
stations.  We  notice  on  the  seismogram  a  faint  widening 
of  the  straight  line  at  20  hrs.  5  min.  2  sec.  (8  hrs.  5  min. 
2  sec.  P.M.).  The  amplitude  of  the  oscillations  then  began 

35 


WORLDS   IN   THE   MAKING 

to  widen,  and  the  heaviest  concussions  were  noticed  at 
20  hrs.  36  min.  25  sec.,  and  20  hrs.  42  min.  49  sec., 
after  which  the  amplitudes  slowly  decreased  with  smaller 
shocks.  The  first  shock  of  20  hrs.  5  min.  2  sec.  is  called 
the  preliminary  tremor.  This  tremor  passes  through  the 
interior  of  the  earth  at  a  velocity  of  propagation  of  9.2 
km.  (5 1  miles)  per  second.  It  would  require  twenty- three 
minutes  to  pass  through  the  earth  along  a  diameter.  The 
tremor  is  very  feeble,  which  is  ascribed  to  the  extraordi- 
narily great  friction  characteristic  of  the  strongly  heated 
gases  which  are  confined  in  the  interior  of  the  earth. 
The  principal  violent  shock  at  20  hrs.  36  min.  25  sec.  was 
caused  by  a  wave  travelling  through  the  solid  crust  of  the 
earth.  The  intensity  of  this  shock  is  much  less  impaired 
than  that  of  the  just-mentioned  tremor,  and  it  travels 
with  the  smaller  velocity  of  about  3.4  km.  (2.1  miles)  along 
the  earth's  surface. 

The  velocity  of  propagation  of  concussion  pulses  has  been 
calculated  for  a  mountain  of  quartz,  in  which  it  would  be 
3.6  km.  (2.2  miles)  per  second,  very  nearly  the  same  as  the 
last-mentioned  figure.  We  should  expect  this,  since  the 
firm  crust  of  the  earth  consists  essentially  of  solid  silicates 
— i.e.,  compounds  of  quartz  endowed  with  similar  prop- 
erties. 

Measured  at  small  distances  from  the  origin,  the  veloc- 
ity of  propagation  of  the  wave  appears  smaller,  and  the 
first  preliminary  tremor  is  frequently  not  observed.  The 
velocity  may  be  diminished  to  2  km.  (1J  miles)  per  second. 
The  reason  is  that  the  pulse  partly  describes  a  curve  in 
the  more  solid  portions  of  the  crust,  and  partly  passes 
through  looser  strata,  through  which  the  wave  travels  at 
a  much  slower  rate  than  in  firm  ground;  for  instance,  at 
1.2  km.  through  loose  sandstones,  at  1.4  km.  through  the 
water  of  the  ocean,  and  at  0.3  km.  through  loose  sand. 

36 


VOLCANIC   PHENOMENA  AND  EARTHQUAKES 

We  recognize  that  it  should  be  possible  to  calculate  the 
distance  between  the  point  of  observation  and  the  origin 
of  the  earthquake  from  the  data  relating  to  the  arrivals 
of  the  first  preliminary  tremor  and  of  the  principal  shock 
of  maximum  amplitude.  The  violent  shock  is  some- 
times repeated  after  a  certain  time,  though  with  de- 
creased intensity.  It  has  often  been  observed  that 
this  secondary,  less  violent,  shock  seems  to  have  trav- 
elled all  round  the  earth  via  the  longest  road  between 
the  origin  and  the  point  of  observation,  just  like  one 
portion  of  the  aerial  waves  in  the  eruption  of  Krakatoa 
(compare  page  27).  The  velocity  of  propagation  of 
this  secondary  shock  is  the  same  as  that  of  the  princi- 
pal shock. 

Milne  has  deduced  from  his  observations  that,  when  the 
line  joining  the  origin  of  the  earthquake  and  the  point  of 
observation  does  not  at  its  lowest  level  descend  deeper 
than  50  km.  below  the  surface  of  the  earth,  the  pulse  will 
travel  undivided  through  the  solid  crust  of  the  earth.  For 
this  reason  we  estimate  the  thickness  of  the  solid  crust  at 
50  km.  The  value  is  in  almost  perfect  agreement  with 
the  one  which  we  had  (on  page  16)  derived  from  the  in- 
crease of  temperature  with  greater  depths.  It  should  fur- 
ther be  mentioned,  perhaps,  that  the  density  of  the  earth 
in  the  vicinity  has  been  determined  from  pendulum  ob- 
servation, and  that  this  density  seems  to  be  rather  varia- 
ble down  to  the  depths  of  50  or  60  km.,  but  to  become 
more  uniform  at  greater  depths.  These  50  or  60  km. 
(31  or  37  miles)  would  belong  to  the  solid  crust  of  tne 
earth. 

The  movement  of  earthquake  shocks  through  the  earth 
thus  teaches  us  that  the  solid  earth-crust  cannot  be  very 
thick,  and  that  the  core  of  the  earth  is  probably  gaseous. 
The  similar  conclusions,  to  which  these  various  consider- 

37 


WORLDS    IN    THE   MAKING 

ations  had  led  us,  may  therefore  come  very  near  the  truth. 
A  careful  study  of  seismograms  may,  we  hope,  help  us  to 
learn  more  about  the  central  portions  of  the  earth,  which 
at  first  sight  appear  to  be  absolutely  inaccessible  to  scien- 
tific research. 


II 

THE  CELESTIAL  BODIES,  IN   PARTICULAR  THE  EARTH, 
AS   ABODES   OF    ORGANISMS 

THERE  is  no  more  elevating  spectacle  than  to  contem- 
plate the  sky  with  its  thousands  of  stars  on  a  clear  night. 
When  we  send  our  thoughts  to  those  lights  glittering  in 
infinite  distance,  the  question  forces  itself  upon  us,  whether 
there  are  not  out  there  planets  like  our  own  that  will  sus- 
tain organic  life.  How  little  interest  do  we  take  in  a  barren 
island  of  the  Arctic  Circle,  on  which  not  a  single  plant  will 
grow,  compared  to  an  island  in  the  tropics  which  is  teem- 
ing with  life  in  its  most  wonderful  variety !  The  unknown 
worlds  occupy  our  minds  much  more  when  we  may  fancy 
them  inhabited  than  when  we  have  to  regard  them  as 
dead  masses  floating  about  in  space. 

We  have  to  ask  ourselves  similar  questions  with  regard 
to  our  own  little  planet,  the  earth.  Was  it  always  covered 
with  verdure,  or  was  it  once  sterile  and  barren?  And  if 
that  be  so,  what  are  the  conditions  under  which  the  earth 
can  fulfil  its  actual  part  of  harboring  organic  life?  That 
"  the  earth  was  without  form"  in  the  beginning  is  unques- 
tionable. It  does  not  matter  whether  we  assume  that  it 
was  once  all  through  an  incandescent  liquid,  which  may 
be  the  most  probable  assumption,  or  that  it  was,  as 
Lockyer  and  Moulton  think,  formed  by  the  accumulation 
of  meteoric  stones  which  became  incandescent  when  ar- 
rested in  their  motion. 

4  39 


WORLDS   IN   THE   MAKING 

We  have  seen  that  the  earth  probably  consists  of  a  mass 
of  gas  encased  within  a  shell  which  is  solid  on  the  outside 
and  remains  a  viscid  liquid  on  the  inner  side.  We  pre- 
sume with  good  reason  that  the  earth  was  originally  a 
mass  of  gas  separated  from  the  sun,  which  is  still  in  the 
same  state.  By  radiation  into  cold  space  the  sphere  of  gas 
which,  on  the  whole,  would  behave  as  our  sun  does  now, 
would  gradually  loss  its  high  temperature,  and  finally  a  solid 
crust  could  form  on  its  surface.  Lord  Kelvin  has  calculated 
that  it  would  not  require  more  than  one  hundred  years 
before  the  temperature  of  this  crust  would  sink  to  100°. 
Supposing,  even,  that  Kelvin's  calculations  should  not 
quite  be  confirmed,  we  may  yet  maintain  that  not  many 
thousands  of  years  would  have  elapsed  from  the  time  when 
the  earth  assumed  its  first  crust  at  about  1000°  till  the 
age  when  this  temperature  had  fallen  below  100°  (212°  F.). 
Living  beings  certainly  could  not  exist  so  long,  since  the 
albumen  of  the  cells  would  at  once  coagulate  at  the  tem- 
perature of  boiling  water,  like  the  white  of  an  egg.  Yet 
it  has  been  reported  that  some  of  the  hot  springs  of  New 
Zealand  contain  algae,  although  at  a  temperature  of  over 
80°.  When  I  went  to  Yellowstone  Park  to  inquire  into  the 
correctness  of  this  statement,  I  found  that  the  algae  existed 
only  at  the  edge  of  the  hot  springs,  where  the  temperature 
did  not  exceed  60°  (140^  yF.).  The  famous  American 
physiologist  Loeb  states  that  we  do  not  meet  with  algae  in 
hot  springs  at  temperatures  above  55°. 

Since,  now,  the  temperature  of  the  earth-crust  would 
much  more  quickly  sink  from  100°  to  55°  than  it  had  fallen 
from  1000°  to  100°,  we  may  imagine  that  only  a  few  thou- 
sands of  years  may  have  intervened  between  the  formation 
of  the  first  crust  of  the  earth  and  the  cooling  down  to  a 
temperature  such  as  would  sustain  life.  Since  that  time  the 
temperature  has  probably  never  been  so  low  that  the 

40 


CELESTIAL   BODIES  AS   ABODES    OF  ORGANISMS 

larger  portion  of  the  earth's  surface  would  not  have  been 
able  to  support  organisms,  although  there  have  been 
several  glacial  ages  in  which  the  arctic  districts  inacces- 
sible to  life  must  have  extended  much  farther  than  at 
present.  The  ocean  will  also  have  been  free  of  ice  over 
much  the  greatest  portion  of  its  surface  at  all  times,  and 
may  therefore  have  been  inhabited  by  organisms  in  all 
ages.  The  interior  of  the  earth  cools  continually,  though 
slowly,  because  heat  passes  from  the  inner,  warmer  por- 
tions to  the  other,  cooler  portions  through  the  crust  of  the 
earth. 

The  earth  is  able  to  serve  as  the  abode  of  living  Beings 
because  its  outer  portions  are  cooled  to  a  suitable  tem- 
perature (below  55°)  by  radiation,  and  because  the  cool- 
ing does  not  proceed  so  far  that  the  open  sea  would  con- 
tinually be  frozen  over,  and  that  the  temperature  on  the 
Continent  would  always  remain  below  freezing-point.  We 
owe  this  favorable  intermediate  stage  to  the  fact  that 
the  radiation  from  the  sun  balances  the  loss  of  heat  by 
radiation  into  space,  and  that  it  is-  capable  of  maintaining 
the  greater  portion  of  the  surface  of  the  earth  at  a  tem- 
perature above  the  freezing-point  of  water.  The  temper- 
ature conditioning  life  on  a  planet  is  therefore  maintained 
only  because,  on  the  one  side,  light  and  heat  are  received 
by  radiation  from  the  sun  in  sufficient  quantities,  while  on 
the  other  side  an  equivalent  radiation  of  heat  takes  place 
into  space.  If  the  heat  gain  and  the  heat  loss  were  not 
to  balance  each  other,  the  term  of  suitable  conditions  would 
not  last  long.  The  temperature  of  the  earth-crust  could 
sink  in  a  few  hundreds  or  thousands  of  years  from  1000° 
to  100°,  because  when  the  earth  was  at  this  high  tempera- 
ture its  radiation  into  space  predominated  over  the  radia- 
tion received  from  the  sun.  On  the  other  hand,  about  a 

hundred  million  years  have  passed,  according  to  Joly,  since 

41 


WORLDS    IN    THE   MAKING 

the  age  when  the  ocean  originated.  The  temperature  of 
the  earth,  therefore,  required  this  long  space  of  time  in  order 
to  cool  down  from  365°  (at  which  temperature  water  vapor 
can  first  be  condensed  to  liquid  water)  to  its  present  tem- 
perature. The  cooling  afterwards  proceeded  at  a  slower 
rate,  because  the  difference  between  the  radiations  inward 
and  outward  was  lessened  with  the  diminishing  tempera- 
ture of  the  earth.  Various  methods  have  been  applied  in 
estimating  these  periods.  Joly  based  his  estimate  on  the 
percentage  of  salt  in  the  sea  and  in  the  rivers.  If  we  cal- 
culate how  much  salt  there  is  in  the  sea,  and  how  much 
salt  the  rivers  can  supply  to  it  in  the  course  of  a  year,  we 
arrive  at  the  result  that  the  quantity  of  salt  now  stored  in 
the  ocean  might  have  been  supplied  in  about  a  hundred 
million  years. 

We  arrive  at  still  higher  numbers  when  we  calculate  the 
time  which  must  have  elapsed  during  the  deposition  of 
all  the  stratified  and  sedimentary  layers.  Sir  Archibald 
Geikie  estimates  the  total  thickness  of  those  strata,  sup- 
posing them  to  have  been  undisturbed,  at  30,000  m. 
(nearly  20  miles).  He  concludes,  further,  from  the  ex- 
amination of  more  recent  strata,  that  every  stratum  one 
metre  in  thickness  must  have  required  from  3000  to  20,000 
years  for  its  formation.  We  should,  therefore,  have  to  al- 
low a  space  of  from  ninety  to  six  hundred  million  years  for 
the  deposition  of  all  the  sedimentary  strata.  The  Finnish 
geologist  Sederholm  even  fixes  the  time  at  a  thousand 
million  years. 

Another  method  again  starts  from  the  consideration 
that,  while  the  temperature  of  the  surface  of  the  earth 
remains  fairly  steady  owing  to  the  heat  exchange  between 
solar  radiation  and  terrestrial  radiation  into  space,  the  in- 
terior of  the  earth  must  have  shrunk  with  the  cooling. 
How  far  this  shrinkage  extends  we  may  estimate  from  the 

42 


CELESTIAL   BODIES   AS   ABODES    OF  ORGANISMS 

formation  of  the  mountain  chains  which,  according  to 
Rudzki,  cover  1.6  per  cent,  of  the  earth's  surface.  The 
earth's  radius  should  consequently  have  contracted  by 
about  0.8  per  cent.,  corresponding  to  a  cooling  through 
about  300°,  which  would  require  two  thousand  million  years. 

Quite  recently  the  renowned  physical  chemist  Ruther- 
ford has  expounded  a  most  original  method  of  estimating 
the  age  of  minerals.  Uranium  and  thorium  are  supposed 
to  produce  helium  by  their  slow  dissociation,  and  we  know 
how  much  helium  is  produced  from  a  certain  quantity  of 
uranium  or  thorium  in  a  year.  Now  Ramsay  has  deter- 
mined the  percentage  of  helium  in  the  uranium  mineral 
fergusonite  and  in  thorianite.  Rutherford  then  calculates 
the  time  which  would  have  passed  since  the  formation  of 
these  minerals.  He  demands  at  least  four  hundred  million 
years,  "for  very  probably  some  helium  has  escaped  from 
the  minerals  during  that  time."  Although  this  estimate 
is  very  uncertain,  it  is  interesting  to  find  that  it  leads  to 
an  age  for  the  solid  earth-crust  of  the  same  order  of  magni- 
tude as  the  other  methods. 

During  this  whole  epoch  of  almost  inconceivable  length 
of  between  one  hundred  million  and  two  thousand  million 
years,  organisms  have  existed  on  the  surface  of  the  earth 
and  in  the  sea  which  do  not  differ  so  very  much  from 
those  now  alive.  The  temperature  of  the  surface  may 
have  been  higher  than  it  is  at  present;  but  the  differ- 
ence cannot  be  very  great,  and  will  amount  to  20°  Cent. 
(36°  F.)  at  the  highest.  The  actual  mean  temperature  of 
the  surface  of  the  earth  is  16°  Cent.  (61°  F.).  It  varies 
from  about  —20°  Cent.  (-4°F.)  at  the  North  Pole,  and 
-10°  Cent.  (  +  14°  F.)  at  the  South  Pole  to  26°  Cent.  (79° 
F.)  in  the  tropical  zone.  The  main  difference  between  the 
temperatures  of  the  earth's  surface  in  the  most  remote 
period  from  which  fossils  are  extant  and  the  actual  state 

43 


WORLDS    IN    THE   MAKING 

rather  seems  to  be  that  the  different  zones  of  the  earth 
are  now  characterized  by  unequal  temperatures,  while  in 
the  remote  epochs  the  heat  was  almost  uniformly  dis- 
tributed over  the  whole  earth. 

The  condition  for  this  prolonged,  almost  stationary  state 
was  that  the  gain  of  heat  of  the  earth's  surface  by  radiation 
from  the  sun  and  the  loss  of  heat  by  radiation  into  space 
nearly  balanced  each  other.  That  the  replenishing  supply 
by  radiation  from  an  intensely  hot  body — in  our  case  the 
sun — is  indispensable  for  the  existence  of  life  will  be  evident 
to  everybody.  Not  everybody  may,  however,  have  con- 
sidered that  the  loss  of  heat  into  cold  space  or  into  colder 
surroundings  is  just  as  indispensable.  To  some  people, 
indeed,  the  assumption  that  the  earth  as  well  as  the  sun 
should  waste  the  largest  portions  of  their  vital  heat  as  radi- 
ation into  cold  space  appears  so  unsatisfactory  that  they  pre- 
fer to  believe  radiation  to  be  confined  to  radiation  between 
celestial  bodies;  there  is  no  radiation  into  space,  in  their 
opinion.  All  the  solar  heat  would  thus  benefit  the  planets 
and  the  moons  in  the  solar  system,  and  only  a  vanishing 
portion  of  it  would  fall  upon  the  fixed  stars,  because  their 
visual  angles  are  so  small.  If  that  were  really  correct,  the 
temperature  of  the  planets  would  rise  at  a  rapid  rate  until 
it  became  almost  equal  to  that  of  the  sun,  and  all  life  would 
become  impossible.  We  are  therefore  constrained  to  ad- 
mit that  "  things  are  best  as  they  arc,"  although  the  great 
waste  of  solar  heat  certainly  weakens  the  solar  energy. 

The  opinion  that  all  the  solar  heat  radiated  into  infinite 
space  is  wasted,  starts  moreover  from  a  hypothesis  which  is 
not  proved,  and  which  is  highly  improbable — namely,  that 
only  an  extremely  small  portion  of  the  sky  is  covered  with 
celestial  bodies.  That  might  certainly  be  correct  if  we 
assumed,  as  has  formerly  been  done,  that  the  majority 
of  the  celestial  bodies  must  be  luminous.  We  do  not 

44 


CELESTIAL   BODIES   AS   ABODES    OF   ORGANISMS 

possess,  however,  any  reliable  knowledge  of  the  number 
and  size  of  the  dark  celestial  bodies.  In  order  to  account 
for  the  observed  movements  of  different  stars,  it  has 
been  thought  that  there  must  be  in  the  neighborhood  of 
some  of  them  dark  stars  of  enormous  size  whose  masses 
would  surpass  the  mass  of  our  sun,  or,  at  least,  be  equal 
to  it.  But  the  largest  number  of  the  dark  celestial  bodies 
which  hide  the  rays  from  the  stars  behind  them  probably 
consist  of  smaller  particles,  such  as  we  observe  in  meteors 
and  in  comets,  and  to  a  large  extent  of  so-called  cosmical 
dust.  The  observations  of  later  years,  by  the  aid  of 
most  powerful  instruments,  have  shown  that  so-called 
nebulse  and  nebulous  stars  abound  throughout  the  heav- 
ens. In  their  interior  we  should  probably  find  accumu- 
lations of  dark  masses. 

The  light  intensity  of  most  of  the  nebulae  is,  moreover, 
far  too  weak  to  permit  of  their  being  perceived.  We 
have,  therefore,  to  imagine  that  there  are  bodies  all 
through  infinite  space,  and  about  as  numerous  as  they 
are  in  .the  immediate  neighborhood  of  our  solar  system. 
Thus  every  ray  from  the  sun,  of  whatever  direction,  would 
finally  hit  upon  some  celestial  body,  and  nothing  would 
be  lost  of  the  solar  radiation,  nor  of  the  stellar  radiation. 

As  regards  the  radiation-heat  exchange,  the  earth 
might  be  likened  to  a  steam-engine.  In  order  that  the 
steam-engine  shall  perform  useful  work,  it  is  necessary 
not  only  that  the  engine  be  supplied  with  heat  of  high 
temperature  from  a  furnace  and  a  boiler,  but  also  that  the 
engine  be  able  to  give  its  heat  up  again  to  a  heat  reservoir 
of  lower  temperature — a  condenser  or  cooler.  It  is  only 
by  transferring  heat  from  a  body  of  higher  temperature 
to  another  body  of  lower  temperature  that  the  engine 
can  do  work.  In  a  similar  way  no  work  can  be  done 
on  the  earth,  and  no  life  can  exist,  unless  heat  be  conferred 

45 


WORLDS    IN    THE   MAKING 


by  the  intermediation  of  the  earth  from  a  hot  body,  the 
sun,  to  the  colder  surroundings  of  universal  space — i.  e., 
to  the  cold  celestial  bodies  in  it. 

To  a  certain  extent  the  temperature  of  the  earth's  sur- 
face, as  we  shall  presently  see,  is  conditioned  by  the  prop- 
erties of  the  atmosphere  surrounding  it,  and  particularly 
by  the  permeability  of  the  latter  for  the  rays  of  heat. 

If  the  earth  did  not  possess  an  atmosphere,  or  if  this 
atmosphere  were  perfectly  diathermal  —  i.e.,  pervious 
to  heat  radiations — we  should  be  able  to  calculate  the 
mean  temperature  of  the  earth's  surface,  given  the  in- 
tensity of  the  solar  radiation,  from  Stefan's  law  of  the 
dependence  of  heat  radiation  bn  its  temperature.  Start- 
ing from  the  not  improbable  assumption  that,  at  a  mean 
distance  of  the  earth  from  the  sun,  the  solar  rays  would 
send  2.5  gramme-calories  per  minute  to  a  body  of  cross 
section  of  1  sq.  centimetre  at  right  angles  to  the  rays  of 
the  sun,  Christiansen  has  calculated  the  mean  tempera- 
tures of  the  surfaces  of  the  various  planets.  The  follow- 
ing table  gives  his  figures,  and  also  the  mean  distances 
of  the  planets  from  the  sun,  in  units  of  the  mean  distance 
of  the  earth  from  the  sun,  149.5  million  kin.  (nearly  93 
million  miles) : 


Planet 

Radius                   Mass 

Mean 
distance 

Mean 
temperature 

Density 
according 
to  See 

According  to  See 

Mercury...  . 
Venus 

0.341 
0.955 
1 
0.273 
0.53 
11.13 
9.35 
3.35 
3.43 
109.1 

0.0224 
O.S15 
1 
0.01228 
0.1077 
317.7 
95.1 
14.6 
17.2 
332,750, 

0.39 
0.72 
1 
1 
1.52 
5.2 
9.55 
19.22 
30.12 
0 

+   17S°(332°) 
+     65° 
+       6.5° 
+       6.5°(105°) 
-     37° 
-   147° 
-   180° 
-  207° 
-  221° 
+  6200° 

0.564 
0.936 

1 
0.604 
0.729 
0.230 
0.116 
0.388 
0.429 
0.256 

Earth  
Moon  
Mars  

Jupiter  
Saturn  .  .  . 
Uranus  .... 
Neptune  .  . 
Sun  

46 


CELESTIAL   BODIES   AS   ABODES    OF  ORGANISMS 

In  the  case  of  Mercury,  I  have  added  another  figure, 
332°.  Mercury  always  turns  the  same  side  to  the  sun, 
and  the  hottest  point  of  this  side  would  reach  a  tempera- 
ture of  397°;  its  mean  temperature,  according  to  my  cal- 
culation, is  332°,  while  the  other  side,  turned  away  from 
the  sun,  cannot  be  at  a  temperature  much  above  absolute 
zero,  —273°.  I  have  made  a  similar  calculation  for  the 
moon,  which  turns  so  slowly  about  its  axis  (once  in 
twenty-seven  days)  that  the  temperature  on  the  side 
illuminated  by  the  sun  remains  almost  as  high  (106°) 
as  if  the  moon  were  always  turning  the  same  face  to  the 
sun.  The  hottest  point  of  this  surface  would  attain  a 
temperature  of  150°,  while  the  poles  of  the  moon  and  that 
part  of  the  other  side  which  remains  longest  without 
illumination  can,  again,  not  be  much  above  absolute  zero 
temperature.  This  estimate  is  in  fair  agreement  with 
the  measurements  made  of  the  lunar  radiation  and  the 
temperature  estimate  based  upon  it.  The  first  measure- 
ment of  this  kind  was  made  by  the  Earl  of  Rosse.  He 
ascertained  that  the  moon  disk  as  illuminated  by  the 
sun — that  is  to  say,  the  full  moon — would  radiate  as 
much  heat  as  a  black  body  of  the  temperature  110°  Cent. 
(230°  F.).  A  later  measurement  by  the  American  Very 
seems  to  indicate  that  the  hottest  point  of  the  moon  is 
at  about  180°,  which  would  be  30°  higher  than  my  esti- 
mate. In  the  cases  of  the  moon  and  of  Mercury,  which 
do  not  possess  any  atmosphere  to  speak  of,  this  calcula- 
tion may  very  fairly  agree  with  the  actual  state  of  affairs. 

The  temperature  of  the  planet  Venus  would  be  about 
65°  Cent.  (149°  F.)  if  its  atmosphere  were  perfectly  trans- 
parent. We  know,  however,  that  dense  clouds,  prob- 
ably of  water  drops,  are  floating  in  the  atmosphere  of 
this  planet,  preventing  us  from  seeing  its  land  and 
water  surfaces.  According  to  the  determinations  made 

47 


WORLDS    IN    THE   MAKING 

by  Zollner  and  others,  Venus  would  reflect  not  less  than 
76  per  cent,  of  the  incident  light  of  the  sun,  and  the  planet 
would  thus  be  as  white  as  a  snow-ball.  The  rays  of  heat 
are  not  reflected  to  the  same  extent.  We  may  estimate 
that  the  portion  of  heat  absorbed  by  the  planet  is  about 
half  the  incident  heat.  The  temperature  of  Venus  will 
therefore  be  reduced  considerably,  but  it  is  partly  aug- 
mented again  by  the  protective  action  of  this  atmosphere. 
The  mean  temperature  of  Venus  may,  hence,  not  differ 
much  from  the  calculated  temperature,  and  may  amount 
to  about  40°  (104°  F.).  Under  these  circumstances  the 
assumption  would  appear  plausible  that  a  very  consid- 
erable portion  of  the  surface  of  Venus,  and  particularly 
the  districts  about  the  poles,  would  be  favorable  to  or- 
ganic life. 

Passing  to  the  earth,  we  find  that  the  temperature- 
reducing  influence  of  the  clouds  must  be  strong.  They 
protect  about  half  of  the  earth's  surfa'ce  (52  per  cent.) 
from  solar  radiation.  But  even  with  a  perfectly  clear 
sky,  not  all  the  light  from  the  sun  really  reaches  the 
earth's  surface;  for  finely  distributed  dust  is  floating 
even  in  the  purest  air.  I  have  estimated  that  this 
dust  would  probably  absorb  17  per  cent,  of  the  solar 
heat.  Clouds  and  dust  would  therefore  together  deprive 
the  earth  of  34  per  cent,  of  the  heat  sent  to  it,  which 
would  lead  to  a  reduction  of  the  temperature  by  about 
28°.  Dust  and  the  water-bubbles  in  the  clouds  also 
prevent  the  radiation  of  heat  from  the  earth,  so  that 
the  total  loss  of  heat  to  be  charged  to  clouds  and  dust 
will  amount  to  about  20°  (36°  F.). 

It  has  now  been  ascertained  that  the  mean  temperature 
of  the  earth  is  16°  (61°  F.),  instead  of  the  calculated  6.5° 
(43.7°  F.).  Deducting  the  20°  due  to  the  influence  of 
dust  and  clouds,  we  obtain  —14°  (7°  F.),  and  the  ob- 

48 


CELESTIAL   BODIES   AS    ABODES   OF   ORGANISMS 

served  temperature  would  therefore  be  higher  than  the 
calculated  by  no  less  than  30°  (54°  F.).  The  discrep- 
ancy is  explained  by  the  heat-protecting  action  of  the 
gases  contained  in  the  atmosphere,  to  which  we  shall 
presently  refer  (page  51). 

There  are  but  few  clouds  on  Mars.  This  planet  is 
endowed  with  an  atmosphere  of  extreme  transparency, 
and  should  therefore  have  a  high  temperature.  Instead 
of  the  temperature  of  -37°  (35°  F.),  calculated,  the 
mean  temperature  seems  to  be  +10°  (  +  50°  F.).  During 
the  winter  large  white  masses,  evidently  snow,  collect 
on  the  poles  of  Mars,  which  rapidly  melt  away  in 
spring  and  change  into  water  that  appears  dark  to  us. 
Sometimes  the  snow-caps  on  the  poles  of  Mars  disappear 
entirely  during  the  Mars  summer;  this  never  happens 
on  our  terrestrial  poles.  The  mean  temperature  of  Mars 
must  therefore  be  above  zero,  probably  about  +10°. 
Organic  life  may  very  probably  thrive,  therefore,  on 
Mars.  It  is,  however,  rather  sanguine  to  jump  at  the 
conclusion  that  the  so-called  canals  of  Mars  prove  its 
being  inhabited  by  intelligent  beings.  Many  people  re- 
gard the  "canals"  as  optical  illusions;  Lowell's  photo- 
graphs, however,  do  not  justify  this  opinion. 

As  regards  the  other  large  planets,  the  temperatures 
which  we  have  calculated  for  them  are  very  low.  This 
calculation  is,  however,  rather  illusory,  because  these 
planets  probably  do  not  possess  any  solid  or  liquid  sur- 
face, but  consist  altogether  of  gases.  Their  densities,  at 
least,  point  in  this  direction.  In  the  case  of  the  inner 
planets,  Mars  and  our  moon  included,  the  density  is  rather 
less  than  that  of  the  earth.  Mercury  stands  last  among 
them,  with  its  specific  gravity  of  0.564.  There  follows  a 
great  drop  in  the  specific  gravities  of  the  outer  large  plan- 
ets. Saturn,  with  a  density  of  0.116,  is  last  in  this  order; 

'49 


WORLDS    IN   THE   MAKING 

the  densities  of  the  two  outermost  planets  lie  somewhat 
higher — by  0.3  or  0.4  about — but  these  last  data  are  very 
uncertain.  Yet  these  figures  are  of  the  same  order  of  mag- 
nitude as  that  assumed  for  the  sun — 0.25 — and  we  be- 
lieve that  the  sun,  apart  from  the  small  clouds,  is  wholly 
a  gaseous  body.  It  is  therefore  probable  that  the  outer 
planets,  including  Jupiter,  will  also  be  gaseous  and  be 
surrounded  by  dense  veils  of  clouds  which  prevent  our 
looking  down  into  their  interior.  That  view  would  con- 
tend against  the  idea  that  these  planets  can  harbor  any 
living  beings.  We  could  rather  imagine  their  moons  to 
be  inhabited.  If  these  moons  received  no  heat  from  their 
planets,  they  would  assume  the  above-stated  tempera- 
tures of  their  central  bodies.  Looked  at  from  our  moon, 
the  earth  appears  under  a  visual  angle,  3.7  times  as  large 
as  that  of  the  sun.  As  the  temperature  of  the  sun  has, 
from  its  radiation,  been  estimated  at  6200°  Cent.,  or 
6500°  absolute,  the  moon  would  receive  as  much  heat 
from  the  earth  as  from  the  sun,  if  the  earth  had  a  tem- 
perature of  about  3100°  Cent.,  or  3380°  absolute.  When 
the  first  clouds  of  water  vapor  were  being  formed  in  the 
terrestrial  atmosphere,  the  earth's  temperature  was 
about  360°,  and  the  radiation  from  the  earth  to  the 
moon  only  about  1.25- thousandth  of  that  of  the  sun.  The 
present  radiation  from  the  earth  does  not  even  attain 
one- twentieth  of  this  value.  It  is  thus  manifest  that 
the  radiation  from  the  earth  does  not  play  any  part  in 
the  thermal  household  of  the  moon. 

The  relations  would  be  quite  different  if  the  earth  had 
the  11.6  times  greater  diameter  of  Jupiter,  or  the  diameter 
of  Saturn,  which  is  9.3  times  greater  than  its  own.  The 
radiation  from  the  earth  to  the  moon  would  then  make 
up  about  a  sixth  or  a  ninth  of  the  actual  solar  radiation, 

taking  the  temperature  of  the  earth's  surface  at  360°. 

50 


CELESTIAL   BODIES  AS  ABODES    OF  ORGANISMS 

We  can  easily  calculate,  further,  that  Jupiter  and  Saturn 
would  radiate  as  much  heat  against  a  moon  at  a  distance 
of  240,000  or  191,000  km.  respectively  (since  the  distance 
of  the  moon  from  the  earth  amounts  to  384,000  km.)  as  the 
sun  sends  to  Mars  —  taking  the  temperature  of  those 
planets  at  360°  Cent.  Now  we  find,  near  Jupiter  as  well 
as  near  Saturn,  moons  at  the  distances  of  126,000  and 
186,000  km.  respectively,  which  are  smaller  than  those 
mentioned,  and  it  is  not  inconceivable  that  these  moons 
receive  from  their  central  bodies  sufficient  heat  to  render 
life  possible,  provided  that  they  be  enveloped  by  a  heat- 
absorbing  atmosphere.  The  conditions  appear  to  be 
less  favorable  for  the  innermost  satellites  of  Jupiter  and 
Saturn.  When  their  planets  are  shining  at  the  maxi- 
mum brilliancy,  their  light  intensity  is  only  a  sixth  or  a 
ninth  of  the  solar  light  intensity,  which  upon  these  satel- 
lites is  itself  only  one-twenty-seventh  or  one-ninetieth  of 
the  intensity  on  the  earth.  During  the  incandescence 
epoch  of  these  planets  their  moons  will  certainly  for 
some  time  have  been  suitable  for  the  development  of  life. 
That  the  atmospheric  envelopes  limit  the  heat  losses 
from  the  planets  had  been  suggested  about  1800  by  the 
great  French  physicist  Fourier.  His  ideas  were  further 
developed  afterwards  by  Pouillet  and  Tyndall.  Their 
theory  has  been  styled  the  hot-house  theory,  because 
they  thought  that  the  atmosphere  acted  after  the  man- 
ner of  the  glass  panes  of  hot-houses.  Glass  possesses 
the  property  of  being  transparent  to  heat  rays  of  small 
wave  lengths  belonging  to  the  visible  spectrum;  but  it  is 
not  transparent  to  dark  heat  rays,  such,  for  instance,  as 
are  sent  out  by  a  heated  furnace  or  by  a  hot  lump 
of  earth.  The  heat  rays  of  the  sun  now  are  to  a  large 
extent  of  the  visible,  bright  kind.  They  penetrate 
through  the  glass  of  the  hot-house  and  heat  the  earth 

51 


WORLDS   IN   THE   MAKING 

under  the  glass.  The  radiation  from  the  earth,  on  the 
other  hand,  is  dark  and  cannot  pass  back  through  the 
glass,  which  thus  stops  any  losses  of  heat,  just  as  an  over- 
coat protects  the  body  against  too  strong  a  loss  of  heat 
by  radiation.  Langley  made  an  experiment  with  a  box, 
which  he  packed  with  cotton-wool  to  reduce  loss  by 
radiation,  and  which .  he .  provided,  on  the  side  turned 
towards  the  sun,  with  a  double  glass  pane.  He  ob- 
served that  the  temperature  rose  to  113°  (235°  F.),  while 
the  thermometer  only  marked  14°  or  15°  (57°  or  59°  F.) 
in  the  shade.  This  experiment  was  conducted  on  Pike's 
Peak,  in  Colorado,  at  an  altitude  of  4200  m.  (13,800  ft.), 
on  September  9, 1881,  at  1  hr.  4  min.  p.  M.,  and  therefore 
at  a  particularly  intense  solar  radiation. 

Fourier  and  Pouillet  now  thought  that  the  atmosphere 
of  our  earth  should  be  endowed  with  properties  resem- 
bling those  of  glass,  as  regards  permeability  of  heat. 
Tyndall  later  proved  this  assumption  to  be  correct.  The 
chief  invisible  constitutents  of  the  air  which  participate 
in  this  effect  are  water  vapor,  which  is  always  found  in  a 
certain  quantity  in  the  air,  and  carbonic  acid,  also  ozone 
and  hydrocarbons.  These  latter  occur  in  such  small 
quantities  that  no  allowance  has  been  made  for  them  so 
far  in  the  calculations.  Of  late,  however,  we  have  been 
supplied  with  very  careful  observations  on  the  per- 
meability to  heat  of  carbonic  acid  and  of  water  vapor. 
With  the  help  of  these  data  I  have  calculated  that  if 
the  atmosphere  were  deprived  of  all  its  carbonic  acid— 
of  which  it  contains  only  0.03  per  cent,  by  volume — the 
temperature  of  the  earth's  surface  would  fall  by  about  21°. 
This  lowering  of  the  temperature  would  diminish  the 
amount  of  water  vapor  in  the  atmosphere,  and  would 
cause  a  further  almost  equally  strong  fall  of  temperature. 

The  examples,  so  far  as  they  go,  demonstrate  that  com- 

52 


CELESTIAL   BODIES   AS   ABODES    OF  ORGANISMS 

paratively  unimportant  variations  in  the  composition  of 
the  air  have  a  very  great  influence.  If  the  quantity  of 
carbonic  acid  in  the  air  should  sink  to  one-half  its  present 
percentage,  the  temperature  would  fall  by  about  4°;  a 
diminution  to  one-quarter  would  reduce  the  temperature 
by  8°.  On  the  other  hand,  any  doubling  of  the  per- 
centage of  carbon  dioxide  in  the  air  would  raise  the  tem- 
perature of  the  earth's  surface  by  4°;  and  if  the  carbon 
dioxide  were  increased  fourfold,  the  temperature  would 
rise  by  8°.  Further,  a  diminution  of  the  carbonic  acid 
percentage  would  accentuate  the  temperature  differences 
between  the  different  portions  of  the  earth,  while  an  in- 
crease in  this  percentage  would  tend  to  equalize  the  tem- 
perature. 

The  question,  however,  is  whether  any  such  tempera- 
ture fluctuations  have  really  been  observed  on  the  sur- 
face of  the  earth.  The  geologists  would  answer:  yes. 
Our  historical  era  was  preceded  by  a  period  in  which  the 
mean  temperature  was  by  2°  (3.6  F.)  higher  than  at 
present..  We  recognize  this  from  the  former  distribution 
of  the  ordinary  hazel-nut  and  of  the  water-nut  (Trapa 
nalans).  Fossil  nuts  of  these  two  species  have  been 
found  in  localities  where  the  plants  could  not  thrive 
in  the  present  climate.  This  age,  again,  was  preceded 
by  an  age  which,  we  are  pretty  certain,  drove  the  inhabi- 
tants of  northern  Europe  from  their  old  abodes.  The 
glacial  age  must  have  been  divided  into  several  periods, 
alternating  with  intervals  of  milder  climates,  the  so- 
called  inter-glacial  periods.  The  space  of  time  which  is 
characterized  by  these  glacial  periods,  when  the  tem- 
perature— according  to  measurements  based  upon  the 
study  of  the  spreading  of  glaciers  in  the  Alps — must 
have  been  about  5°  (8°  F.)  lower  than  now,  has  been 

estimated  by  geologists  at  not  less  than  100,000  years. 

53 


WORLDS   IN   THE   MAKING 

This  epoch  was  preceded  by  a  wanner  age,  in  which  the 
temperature,  to  judge  from  fossilized  plants  of  those 
days,  must  at  times  have  been  by  8°  or  9°  (14°  or  16°  F.) 
higher  than  at  present,  and,  moreover,  much  more  uni- 
formly distributed  over  the  whole  earth  (Eocene).  Pro- 
nounced fluctuations  of  this  kind  in  the  climate  have 
also  occurred  in  former  geological  periods. 

Are  we  now  justified  in  supposing  that  the  percentage 
of  carbon  dioxide  in  the  air  has  varied  to  an  extent  suffi- 
cient to  account  for  the  temperature  changes?  This 
question  has  been  answered  in  the  affirmative  by  Hog- 
bom,  and,  in  later  times,  by  Stevenson.  The  actual 
percentage  of  carbonic  acid  in  the  air  is  so  insignificant 
that  the  annual  combustion  of  coal,  which  has  now  (1904) 
risen  to  about  900  million  tons  and  is  rapidly  increas- 
ing,1 carries  about  one-seven-hundredth  part  of  its  per- 
centage of  carbon  dioxide  to  the  atmosphere.  Although 
the  sea,  by  absorbing  carbonic  acid,  acts  as  a  regulator 
of  huge  capacity,  which  takes  up  about  five-sixths  of  the 
produced  carbonic  acid,  we  yet  recognize  that  the  slight 
percentage  of  carbonic  acid  in  the  atmosphere  may  by 
the  advances  of  industry  be  changed  to  a  noticeable  de- 
gree in  the  course  of  a  few  centuries.  That  would  imply 
that  there  is  no  real  stability  in  the  percentage  of  carbon 
dioxide  in  the  air,  which  is  probably  subject  to  consider- 
able fluctuations  in  the  course  of  time. 

Volcanism  is  the  natural  process  by  which  the  greatest 
amount  of  carbonic  acid  is  supplied  to  the  air.  Large 
quantities  of  gases  originating  in  the  interior  of  the  earth 
are  ejected  through  the  craters  of  the  volcanoes.  These 
gases  consist  mostly  of  steam  and  of  carbon  dioxide,  which 
have  been  liberated  during  the  slow  cooling  of  the  silicates 

1  It  amounted  in  1890  to  510  million  tons;  in  1894,  to  550;  in 
1899,  to  690;  and  in  1904,  to  890  million  tons. 

54 


CELESTIAL   BODIES  AS   ABODES    OF   ORGANISMS 

in  the  interior  of  the  earth.  The  volcanic  phenomena 
have  been  of  very  unequal  intensity  in  the  different  phases 
of  the  history  of  the  earth,  and  we  have  reason  to  surmise 
that  the  percentage  of  carbon  dioxide  in  the  air  was 
considerably  greater  during  periods  of  strong  volcanic 
activity  than  it  is  now,  and  smaller  in  quieter  periods. 
Professor  Freeh,  of  Breslau,  has  attempted  to  demon- 
strate that  this  would  be  in  accordance  with  geological 
experience,  because  strongly  volcanic  periods  are  dis- 
tinguished by  warm  climates,  and  periods  of  feeble  vol- 
canic intensity  by  cold  climates.  The  ice  age  in  particular 
was  characterized  by  a  nearly  complete  cessation  of  vol- 
canism,  and  the  two  periods  at  the  commencement  and 
at  the  middle  of  the  Tertiary  age  (Eocene  and  Miocene) 
which  showed  high  temperatures  were  also  marked  by 
an  extraordinarily  developed  volcanic  activity.  "This 
parallelism  can  be  traced  back  into  more  remote  epochs. 
It  may  possibly  be  a  matter  of  surprise  that  the  per- 
centage of  carbon  dioxide  in  the  atmosphere  should  not 
constantly  be  increased,  since  volcanism  is  always  pour- 
ing out  more  carbon  dioxide  into  our  atmosphere.  There 
is,  however,  one  factor  which  always  tends  to  reduce  the 
carbon  dioxide  of  the  air,  and  that  is  the  weathering  of 
minerals.  The  rocks  which  were  first  formed  by  the  con- 
gelation of  the  volcanic  masses  (the  so-called  magma)  con- 
sist of  compounds  of  silicic  acid  with  alumina,  lime,  mag- 
nesia, some  iron  and  sodium.  These  rocks  were  gradu- 
ally decomposed  by  the  carbonic  acid  contained  in  the 
air  and  in  the  water,  and  it  was  especially  the  lime, 
the  magnesia,  and  the  alkalies,  arid,  in  some  measure  also 
the  iron,  which  formed  soluble  carbonates.  These  car- 
bonates were  carried  by  the  rivers  down  into  the  seas. 
There  lime  and  magnesia  were  secreted  by  the.  animals 
and  by  the  algse,  and  their  carbonic  acid  became  stored 
s  55 


WORLDS   IN  THE   MAKING 

up  in  the  sedimentary  strata.  Hogbom  estimates  that  the 
limestones  and  dolomites  contain  at  least  25,000  times 
more  carbonic  acid  than  our  atmosphere.  Chamberlin  has 
arrived  at  nearly  the  same  figure — from  20,000  to  30,000 ; 
he  does  not  allow  for  the  precambrian  limestones.  These 
estimates  are  most  likely  far  too  low.  All  the  carbonic 
acid  that  is  stored  up  in  sedimentary  strata  must  have 
passed  through  the  atmosphere.  Another  process  which 
withdraws  carbonic  acid  from  the  air  is  the  assimilation 
of  plants.  Plants  absorb  carbonic  acid  under  secretion 
of  carbon  compounds  and  under  exhalation  of  oxygen. 
Like  the  weathering,  the  assimilation  increases  with  the 
percentage  of  carbonic  acid.  The  Polish  botanist  E. 
Godlewski  showed  as  early  as  1872  that  various  plants  (he 
studied  Typha  latifolia  and  Glyceria  spectabilis  with  par- 
ticular care)  absorb  from  the  air  an  amount  of  carbonic 
acid  which  increases  proportionally  with  the  percentage 
of  carbonic  acid  in  the  atmosphere  up  to  1  per  cent.,  and 
that  the  assimilation  then  attains,  in  the  former  plant,  a 
maximum  at  6  per  cent.,  and  in  the  latter  plant  at  9  per 
cent.  The  assimilation  afterwards  diminishes  if  the  car- 
bonic acid  percentage  is  further  augmented.  If,  therefore, 
the  percentage  of  carbon  dioxide  be  doubled,  the  absorp- 
tion by  the  plants  would  also  be  doubled.  If,  at  the  same 
time,  the  temperature  rises  by  4°,  the  vitality  will  increase 
in  the  ratio  of  1  :  1.5,  so  that  the  doubling  of  the  carbon 
dioxide  percentage  will  lead  to  an  increase  in  the  ab- 
sorption of  carbonic  acid  by  the  plant  approximately  in 
the  ratio  of  1  :  3.  The  same  may  be  assumed  to  hold  for 
the  dependence  of  the  weathering  upon  the  atmospheric 
percentage  of  carbonic  acid.  An  increase  of  the  carbon 
dioxide  percentage  to  double  its  amount  may  hence  be 
able  to  raise  the  intensity  of  vegetable  life  and  the  in- 
tensity of  the  inorganic  chemical  reactions  threefold. 

56 


CELESTIAL   BODIES   AS   ABODES    OF  ORGANISMS 

According  to  the  estimate  of  the  famous  chemist  Lie- 
big,  the  quantity  of  organic  matter  (freed  of  water)  which 
is  produced  by  one  hectare  (2.5  acres)  of  soil,  meadow- 
land,  or  forest  is  nearly  the  same,  approximately  2.5 
tons  per  year  in  central  Europe.  In  many  parts  of  the 
tropics  the  growth  is  much  more  rapid;  in  other  places, 
in  the  deserts  and  arctic  regions,  much  more  feeble.  We 
may  be  justified  in  accepting  Liebig's  figure  as  an  average 
for  the  firm  land  on  our  earth.  Of  the  organic  substances 
to  which  we  have  referred,  and  which  mainly  consist  of 
cellulose,  carbon  makes  up  40  per  cent.  Thus  the  actual 
annual  carbon  production  by  plants  would  amount  to 
13,000  million  tons — i.  e.,  not  quite  fifteen  times  more 
than  the  consumption  of  coal,  and  about  one-fiftieth  of 
the  quantity  of  the  carbon  dioxide  in  the  air.  If,  there- 
fore, all  plants  were  to  deposit  their  carbon  in  peat-bogs, 
the  air  would  soon  be  depleted  of  its  carbon  dioxide. 
But  it  is  only  a  fraction  of  one  per  cent,  of  the  coal 
which  is  produced  by  plants  that  is  stored  up  for  the 
future  in  this  way.  The  rest  is  sent  back  into  the  atmos- 
phere by  combustion  or  by  decay. 

Chamberlin  relates  that,  together  with  five  other 
American  geologists,  he  attempted  to  estimate  how  long 
a  time  would  be  required  before  the  carbon  dioxide  of 
the  air  would  be  consumed  by  the  weathering  of  rocks. 
Their  various  estimates  yielded  figures  ranging  from  5000 
to  18,000  years,  with  a  probable  average  of  10,000  years. 
The  loss  of  carbonic  acid  by  the  formation  of  peat  may 
be  estimated  at  the  same  figure.  The  production  of  car- 
bonic acid  by  the  combustion  of  coal  would  therefore 
suffice  to  cover  the  loss  of  carbonic  acid  by  weathering 
and  by  peat  formation  seven  times  over.  Those  are 
the  two  chief  factors  deciding  the  consumption  of  car- 
bonic acid,  and  we  thus  recognize  that  the  percentage  of 

57 


WORLDS   IN    THE   MAKING 

carbonic  acid  in  the  air  must  be  increasing  at  a  constant 
rate  as  long  as  the  consumption  of  coal,  petroleum,  etc., 
is  maintained  at  its  present  figure,  and  at  a  still  more 
rapid  rate  if  this  consumption  should  continue  to  increase 
as  it  does  now. 

This  consideration  enables  us  to  picture  to  ourselves 
the  possibility  of  the  enormous  plant-growth  which  must 
have  characterized  certain  geological  periods  of  our  earth 
— for  instance,  the  carboniferous  period. 

This  period  is  known  to  us  from  the  extraordinarily 
large  number  of  plants  which  we  find  embedded  in  the 
clay  of  the  swamps  of  those  days.  Those  plants  were 
slowly  carbonized  afterwards,  and  their  carbon  is  in  our 
age  returned  to  its  original  place  in  the  household  of 
nature  in  the  shape  of  carbonic  acid.  A  great  portion  of 
the  carbonic  acid  has  disappeared  from  the  atmosphere 
of  the  earth,  and  has  been  stored  up  as  coal,  lignite,  peat, 
petroleum,  or  asphalt  in  the  sedimentary  strata.  Oxygen 
was  liberated  at  the  same  time,  and  passed  into  our  at- 
mospheric sea.  It  has  been  calculated  that  the  amount 
of  oxygen  in  the  air — 1216  billion  tons — approximately 
corresponds  to  the  mass  of  fossil  coal  which  is  stored  up 
in  the  sedimentary  strata.  The  supposition  appears  nat- 
ural, therefore,  that  all  the  oxygen  of  the  air  may  have 
been  formed  at  the  expense  of  the  carbonic  acid  in  the  air. 
This  view  was  first  advanced  by  Kcehne,  of  Brussels,  in 
1856,  and  later  discussions  have  strengthened  its  probabil- 
ity. Part  of  the  oxygen  is  certainly  consumed  by  weath- 
ering processes,  and  absorbed — e.  g.,  by  sulphides  and  by 
ferro-salts;  without  this  oxidation  the  actual  quantity  of 
oxygen  in  the  air  would  be  greater.  On  the  other  hand, 
there  are  in  the  sedimentary  strata  many  oxidizable  com- 
pounds— e.  g.,  especially  iron  sulphides — which  have  prob- 
ably been  reduced  by  the  interaction  of  carbon  (by  or- 

58 


CELESTIAL   BODIES   AS   ABODES    OF  ORGANISMS 

ganic  compounds).  A  large  number  of  the  substances 
which  consume  oxygen  during  their  decomposition  and 
decay  have  also  been  produced  by  the  intermediation  of 
the  coal  which  had  previously  been  deposited  under  lib- 
eration of  oxygen,  so  that  these  substances  are,  by  their 
oxidation,  restored  to  their  original  state.  We  may  hence 
take  it  as  established  that  the  masses  of  free  oxygen  in  the 
air  and  of  free  carbon  in  the  sedimentary  strata  approxi- 
mately correspond  to  each  other,  and  that  probably  all 
the  oxygen  of  the  atmosphere  owes  its  existence  to  plant 
life.  This  appears  plausible  also  for  another  reason. 
We  know  for  certain  that  there  is  some  free  oxygen  in  the 
atmosphere  of  the  sun,  and  that  hydrogen  abounds  in 
the  sun.  The  earth's  atmosphere  may  originally  have 
been  in  the  same  condition.  When  the  earth  cooled 
gradually,  hydrogen  and  oxygen  combined  to  water,  but 
an  excess  of  hydrogen  must  have  remained.  The  pri- 
meval atmosphere  of  the  earth  may  also  have  contained 
hydrocarbons,  as  they  play  an  important  part  in  the 
gases  of  comets.  To  these  gases  there  were  added  car- 
bonic acid  and  water  vapor,  coming  from  the  interior  of 
the  earth.  Thanks  to  its  chemical  inertia,  the  nitrogen 
of  the  air  may  not  have  undergone  much  change  in  the 
course  of  the  ages.  An  English  chemist,  Phipson,  claims 
to  have  shown  that  both  higher  plants  (the  corn-bind) 
and  lower  organisms  (various  bacteria)  can  live  and 
develop  in  an  atmosphere  devoid  of  oxygen  when  it 
contains  carbonic  acid  and  hydrogen.  It  is  also  possible 
that  simple  forms  of  vegetable  life  existed  before  the  air 
contained  any  oxygen,  and  that  these  plants  liberated 
the  oxygen  from  the  carbonic  acid  exhaled  by  the  craters. 
This  oxygen  gradually  (possibly  under  the  influence  of 
electric  discharges)  converted  the  hydrogen  and  the  hydro- 
carbons of  the  air  into  water  and  carbonic  acid  until  those 

59 


WORLDS    IN   THE   MAKING 

elements  were  consumed.  The  oxygen  remained  in  the 
air,  whose  composition  gradually  approached  more  the 
actual  state.1 

This  oxygen  is  an  essential  element  for  the  production 
of  animal  life.  As  animal  life  stands  above  vegetable  life, 
so  animal  life  could  only  originate  at  a  later  stage  than 
plant  life.  Plants  require,  in  addition  to  suitable  tem- 
perature, only  carbonic  acid  and  water,  and  these  gases 
will  probably  be  found  in  the  atmospheres  of  all  the 
planets  as  exhalations  of  their  inner  incandescent  masses 
which  are  slowly  cooling.  The  presence  of  water  vapor 
has  directly  been  established,  by  means  of  the  spectro- 
scope, in  the  atmospheres  of  other  planets — Venus,  Jupi- 
ter, and  Saturn — and  indirectly  by  the  observation  of  a 
snow-cap  on  Mars.  The  spectroscope  further  gives  us 
indication  of  the  presence  of  other  gases.  There  is  an 
intense  band  in  the  red  part  of  the  spectra  of  Jupiter  and 

1  According  to  the  opinion  of  a  colleague  of  mine,  a  botanist,  the 
results  of  the  experiments  of  Phipson  must  be  regarded  as  very  doubt- 
ful, and  some  oxygen  would  appear  to  be  indispensable  for  the  growth 
of  plants.  We  have  to  imagine  the  development  somewhat  as  follows: 
As  the  earth  separated  from  the  solar  nebula,  its  temperature  was  very 
high  at  first  in  its  outer  portions.  At  this  temperature  it  was  not  able 
to  retain  the  lighter  gases,  like  hydrogen  and  helium,  for  a  long  period; 
the  heavy  gases,  like  nitrogen  and  oxygen,  remained.  The  original 
excess  of  hydrogen  and  helium  disappeared,  therefore,  before  the  crust 
of  the  earth  had  been  formed,  and  the  atmosphere  of  the  earth  im- 
mediately after  the  formation  of  the  crust  contained  some  oxygen, 
besides  much  nitrogen,  carbonic  acid,  and  water  vapor.  The  main 
bulk  of  the  actual  atmospheric  oxygen  would  therefore  have  been 
reduced  from  carbon  dioxide  by  the  intermediation  of  plants.  The 
view  that  celestial  bodies  may  lose  part  of  their  atmosphere  is  due  to 
Johnstone  Stoney.  The  atmospheric  gases  escape  the  more  rapidly 
the  lighter  their  molecules  and  the  smaller  the  mass  of  the  celestial 
bodies.  On  these  lines  we  explain  that  the  smaller  celestial  bodies 
like  the  moon  and  Mercury,  have  lost  almost  all  their  atmosphere, 
while  the  earth  lias  only  lost  hydrogen  and  helium,  which  again  have 
been  retained  by  the  sun. 

60 


CELESTIAL   BODIES   AS  ABODES    OF  ORGANISMS 

Saturn,  of  wave-length  0.000618  mm.  Other  new  con- 
stituents of  unknown  nature  have  been  discerned  in  the 
spectra  of  Uranus  and  Neptune.  On  the  other  hand, 
there  is  hardly  any,  or  at  any  rate  only  a  quite  insignifi- 
cant, atmosphere  on  the  moon  and  on  Mercury.  This  is 
easily  understood.  The  temperature  on  that  side  of  Mer- 
cury which  is  turned  away  from  the  sun  is  near  absolute 
zero.  All  the  gases  of  the  planetary  atmosphere  would 
collect  and  condense  there.  If,  then,  Mercury  had  orig- 
inally an  atmosphere,  it  must  have  lost  it  as  it  lost  its 
own  rotation,  compelling  it  to  turn  always  the  same  face 
towards  the  sun.  Similar  reasons  may  account  for  the 
absence  of  a  lunar  atmosphere.  If  Venus  should  like- 
wise always  turn  the  same  side  towards  the  sun,  as  many 
astronomers  assert,  Venus  should  not  have  any  notable 
atmosphere,  nor  clouds  either.  We  know,  however,  that 
this  planet  is  surrounded  by  a  very  marked  developed 
atmosphere.1 

And  that  is  the  strongest  objection  to  the  assumption 
that  Venus  follows  the  example  of  Mercury  as  regards  the 
rotation  about  its  own  axis. 

Since,  now,  warm  ages  have  alternated  with  glacial 
periods,  even  after  man  appeared  on  the  earth,  we  have 
to  ask  ourselves :  Is  it  probable  that  we  shall  in  the  com- 
ing geological  ages  be  visited  by  a  new  ice  period  that 
will  drive  us  from  our  temperate  countries  into  the  hotter 
climates  of  Africa?  There  does  not  appear  to  be  much 
ground  for  such  an  apprehension.  The  enormous  com- 
bustion of  coal  by  our  industrial  establishments  suffices 
to  increase  the  percentage  of  carbon  dioxide  in  the  air  to 
a  perceptible  degree.  Volcanism,  whose  devastations — 

1  That  results  from  the  very  strong  refraction  which  light  undergoes 
in  the  atmosphere  of  Venus  when  this  planet  is  seen  in  front  of  the 
sun's  edge  during  the  so-called  Venus  transits. 

61 


Fig.  17. — Photograph  of  the  surface  of  the  moon,  in  the  vicinity  of 
the  crater  of  Copernicus.  Taken  at  the  Yerkes  Observatory,  Chi- 
cago, U.  S.  A.  Scale:  Diameter  of  moon,  0.55  m.=21.7  in.  Owing 
to  the  absence  of  an  atmosphere  and  of  atmospheric  precipitations, 
the  precipitous  walls  of  the  crater  and  other  elevations  do  not  in- 
dicate any  signs  of  decay 

62 


CELESTIAL   BODIES  AS  ABODES    OF  ORGANISMS 

on  Krakatoa  (1883)  and  Martinique  (1902) — have  been 
terrible  in  late  years,  appears  to  be  growing  more  intense. 
It  is  probable,  therefore,  that  the  percentage  of  carbonic 
acid  increases  at  a  rapid  rate.  Another  circumstance 
points  in  the  same  direction;  that  is,  that  the  sea  seems 
to  withdraw  carbonic  acid  from  the  air.  For  the  carbonic 
acid  percentage  above  the  sea  and  on  islands  is  on  an 
average  10  per  cent,  less  than  the  above  continents. 

If  the  carbonic  acid  percentage  of  the  air  had  kept 
constant  for  ages,  the  percentage  of  the  water  would 
have  found  time  to  get  into  equilibrium  with  it;  but 
the  sea  actually  absorbs  carbonic  acid  from  the  air. 
Thus  the  sea-water  must  have  been  in  equilibrium  with 
an  atmosphere  which  contained  less  carbonic  acid  than 
the  present  atmosphere.  Hence  the  carbonic  acid  per- 
centage has  been  increasing  of  late. 

We  often  hear  lamentations  that  the  coal  stored  up  in 
the  earth  is  wasted  by  the  present  generation  without 
any  thought  of  the  future,  and  we  are  terrified  by  the 
awful  destruction  of  life  and  property  which  has  followed 
the  volcanic  eruptions  of  our  days.  We  may  find  a  kind 
of  consolation  in  the  consideration  that  here,  as  in  every 
other  case,  there  is  good  mixed  with  the  evil.  By  the 
influence  of  the  increasing  percentage  of  carbonic  acid 
in  the  atmosphere,  we  may  hope  to  enjoy  ages  with  more 
equable  and  better  climates,  especially  as  regards  the 
colder  regions  of  the  earth,  ages  when  the  earth  will  bring 
forth  much  more  abundant  crops  than  at  present,  for  the 
benefit  of  rapidly  propagating  mankind. 


Ill 

RADIATION   AND  CONSTITUTION   OF  THE   SUN 

THE  question  has  often  been  discussed  in  past  ages, 
and  again  in  the  last  century,  in  how  far  the  position  of 
our  earth  within  the  solar  system  may  be  regarded  as 
secure.  One  might  apprehend  two  things.  Either  the 
distance  of  the  earth  from  the  sun  might  increase  or 
decrease,  or  the  rotation  of  the  earth  about  its  axis 
might  be  arrested;  and  either  of  these  possibilities  would 
threaten  the  continuance  of  life  on  the  earth.  The 
problem  of  the  stability  of  the  solar  system  has  been 
investigated  by  the  astronomers,  and  their  patrons  have 
offered  high  prizes  for  a  solution  of  the  problem.  If 
the  solar  system  consisted  merely  of  the  sun  and  the 
earth,  the  earth's  existence  would  be  secure  for  ages; 
but  the  other  planets  exercise  a  certain,  though  small, 
influence  upon  the  movements  of  the  earth.  That  this 
influence  can  only  be  of  slight  importance  is  due  to  the 
fact  that  the  total  mass  of  all  the  planets  does  not  ag- 
gregate more  than  one-seven-hundred-and-fiftieth  of  the 
mass  of  the  sun,  and,  further,  to  the  fact  that  the  planets 
all  move  in  nearly  circular  orbits  around  the  centre,  the 
sun,  so  that  they  never  approach  one  another  closely. 
The  calculations  of  the  astronomers  demonstrate  that 
the  disturbances  of  the  earth's  orbit  are  merely  periodi- 
cal, representing  long  cycles  of  from  50,000  to  2,000,000 
years.  Thus  the  whole  effect  is  limited  to  a  slight  vacil- 

(34 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

lation  of  the  orbits  of  the  planets  about  their  mean  posi- 
tions. 

So  far  everything  is  well  and  good.  But  our  solar 
system  is  traversed  by  other  celestial  bodies,  mostly  of 
unknown,  but  certainly  not  of  circular  orbits — namely, 
the  comets.  The  fear  of  a  collision  with  a  comet  still 
alarmed  the  thinkers  of  the  past  century.  Experience 
has,  however,  taught  us  that  collisions  between  the 
earth  and  comets  do  not  lead  to  any  serious  con- 
sequence. The  earth  has  several  times  passed  through 
the  tails  of  comets — for  instance,  in  1819  and  1861— 
and  it  was  only  the  calculating  astronomer  who  became 
aware  of  the  fact.  Once  on  such  an  occasion  we  have 
thought  that  we  observed  a  glow  like  that  of  an  aurora 
in  the  sky.  When  the  earth  was  drawing  near  the 
denser  parts  of  the  comet,  particles  fell  on  the  earth 
in  the  shape  of  showers  of  shooting-stars,  without  doing 
any  appreciable  damage.  The  mass  of  comets  is  too 
small  perceptibly  to  disturb  the  paths  of  the  planets. 

The  rotation  of  the  earth  about  its  axis  should  slowly 
be  diminished  by  the  effects  of  the  tides,  since  they  act 
like  a  brake  applied  to  the  surface  of  the  earth.  This  re- 
tardation is,  however,  so  unimportant  that  the  astronomers 
have  not  been  able  to  establish  it  in  historical  times.  The 
slow  shrinkage  of  the  earth  somewhat  counteracts  this 
effect.  Laplace  believed  that  we  were  able  to  deduce, 
from  an  analysis  of  the  observations  of  solar  eclipses  in 
ancient  centuries,  that  the  length  of  the  day  had  not 
altered  by  more  than  0.01  second  since  the  year  729 
B.C. 

We  know  that  the  sun,  ^accompanied  by  its  planets, 
is  moving  in  space  towards  the  constellation  of  Hercules 
with  a  velocity  of  20  km.  (13  miles)  per  second,  which  is 
amazing  to  our  terrestrial  conceptions.  Possibly  the  con- 

65 


WORLDS    IN    THE   MAKING 

stituents  of  our  solar  system  might  collide  with  some  other 
unknown  celestial  body  on  this  journey.  But  as  the  celes- 
tial bodies  are  sparsely  distributed,  we  may  hope  that  many 
billions  of  years  will  elapse  before  such  a  catastrophe  will 
take  place. 

In  mechanical  respects  the  stability  of  our  system  ap- 
peared to  be  well  established.  Since  the  modern  theory 
of  heat  has  made  its  triumphant  entry  into  natural  science, 
however,  the  aspect  of  matters  has  changed.  We  are 
convinced  that  all  life  and  all  motion  on  the  earth  can 
be  traced  back  to  solar  radiation.  The  tidal  motions  alone 
make  a  rather  unimportant  exception.  We  have  to  ask 
ourselves:  Will  not  the  store  of  energy  in  the  sun,  which 
goes  out,  not  only  to  the  planets,  but  to  a  far  greater 
extent  into  unknown  domains  of  cold  space,  come  to  an 
end,  and  will  not  that  be  the  end  of  all  the  joys  and  sor- 
rows of  earthly  existence?  The  position  appears  des- 
perate when  we  consider  that  only  one  part  in  2300  mill- 
ions of  the  solar  radiation  benefits  the  earth,  and  perhaps 
ten  times  as  much  the  whole  system,  with  all  its  moons. 
The  solar  radiation  is  so  powerful  that  every  gramme  of 
the  mass  of  the  sun  loses  two  calories  in  the  course  of  a 
year.  If,  therefore,  the  specific  heat  of  the  sun  were  the 
same  as  that  of  water,  which  in  this  respect  surpasses 
most  other  substances,  the  solar  temperature  would  fall 
by  2°  Cent.  (3.6°  F.)  every  year.  As,  now,  the  tempera- 
ture of  the  sun  in  its  outer  portion  has  been  estimated  at 
from  6000°  to  7000°,  the  sun  should  have  cooled  complete- 
ly within  historical  times.  And  though  the  interior  of 
the  sun  most  probably  has  a  vastly  higher  temperature 
than  the  outer  portions  which  we  can  observe,  we  should, 
all  the  same,  have  to  expect  that  the  solar  temperature 
and  radiation  would  noticeably  have  diminished  in  his- 
torical times.  But  all  the  documents  from  ancient  Baby- 

66 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

Ion  and  Egypt  seem  to  point  out  that  the  climate  at  the 
dawn  of  historical  times  was  in  those  countries  nearly  the 
same  as  at  present,  and  that,  therefore,  the  sun  shone 
over  the  most  ancient  representatives  of  culture  in  the 
same  way  as  it  shines  on  their  descendants  now. 

The  thesis  has  frequently  been  advanced,  therefore,  that 
the  sun  has  in  its  heat  balance  not  only  an  expenditure  side, 
but  also  an  almost  equally  substantial  income  side.  The 
German  physician  R.  Mayer,  who  has  the  immortal  merit 
of  first  having  given  expression  to  the  conception  of  a 
relation  between  heat  and  mechanical  work,  directed  his 
attention  also  to  the  household  of  the  sun.  He  sug- 
gested that  swarms  of  meteorites,  rushing  into  the  sun 
with  an  amazing  velocity  (of  over  600  km.  per  second), 
would,  when  stopped  in  their  motion,  generate  heat  at 
the  rate  of  45  million  calories  per  gramme  of  meteorites. 
In  future  ages  it  would  be  the  turn  of  the  planets  to  sus- 
tain for  some  time  longer  the  spark  of  life  in  the  sun,  by 
the  sacrifice  of  their  own  existences.  The  sun  would  there- 
fore, like  the  god  Saturn,  have  to  devour  its  own  children 
in  order  to  continue  its  existence.  Of  how  little  avail 
that  would  be  we  learn  from  the  consideration  that  the 
fall  of  the  earth  into  the  sun  would  not  be  able  to  pro- 
long the  heat  expenditure  of  the  sun  by  as  many  as  a  hun- 
dred years.  By  their  rush  into  the  sun,  almost  uniformly 
from  all  sides,  the  meteorites  would,  moreover,  long  since 
have  put  a  stop  to  the  rotation  of  the  sun  about  its  axis. 
Further,  by  virtue  of  the  increasing  mass  and  the  hence 
augmenting  attraction  of  the  sun,  the  length  of  our  year 
would  have  had  to  diminish  by  about  2.8  seconds  per  year, 
which  is  in  absolute  contradiction  to  the  observations  of  the 
astronomers.  According  to  Mayer's  thesis,  a  correspond- 
ing number  of  meteorites  would,  finally,  also  have  to 
tumble  upon  the  surface  of  the  earth,  and  (according  to 

67 


WORLDS    IN   THE   MAKING 

data  which  will  be  furnished  in  Chapter  IV.)  they  should 
raise  the  surface  temperature  to  about  800°.  The  thesis 
is  therefore  misleading. 

We  must  look  for  another  explanation.  It  occurred  to 
Helmholtz,  one  of  the  most  eminent  investigators  in  the 
domain  of  the  mechanical  theory  of  heat,  that,  instead  of 
the  meteorites,  parts  of  the  sun  itself  might  fall  towards 
its  centre,  or,  in  other  words,  that  the  sun  was  shrinking. 
Owing  to  the  high  gravitation  of  the  sun  (27.4  times 
greater  than  on  the  surface  of  the  earth),  the  shrinkage 
would  liberate  a  great  amount  of  heat.  Helmholtz  cal- 
culated that,  in  order  to  cover  the  heat  expenditure  of 
the  sun,  a  shrinkage  of  its  diameter  by  60  m.  annually 
would  be  required.  If  the  sun's  diameter  should  only 
be  diminished  by  one  -  hundredth  of  one  per  cent. — a 
change  which  we  should  not  be  able  to  establish — the 
heat  loss  would  be  covered  for  more  than  2000  years. 
That  seems  at  first  satisfactory.  But  if  we  proceed 
with  our  estimate,  we  find  that  if  the  sun  went  on 
losing  as  much  heat  as  at  present  for  seventeen  million 
years  it  would  have  to  contract  within  this  period 
to  a  quarter  of  its  present  volume,  and  would  there- 
fore acquire  a  density  like  that  of  the  earth.  Long 
before  that,  however,  the  radiation  from  the  sun  would 
have  been  decreased  so  powerfully  that  the  tempera- 
ture on  the  earth's  surface  would  no  longer  rise  above 
freezing-point.  Helmholtz,  on  this  argument,  limited 
the  further  existence  of  the  earth  to  about  six  million 
years.  That  is  less  satisfactory.  But  we  know  noth- 
ing of  the  future  and  must  be  content  with  possibili- 
ties. Not  so,  however,  if  we  calculate  back  with  the  aid 
of  Helmholtz' s  theory.  According  to  this  theory,  and 
according  to  Helmholtz 's  own  data,  a  state  like  the  pres- 
ent cannot  have  existed  for  more  than  ten  million  years. 


RA^fATION   AND   CONSTITUTION   OF  THE   SUN 

Since,  now,  geologists  have  come  to  the  conclusion  that 
the  petrefactions  which  we  find  in  the  fossil-bearing  strata 
of  the  earth  have  needed  at  least  a  hundred  million  years 
for  their  formation,  and  more  probably  a  thousand  million 
years,  and  since,  moreover,  the  still  more  ancient  forma- 
tions— the  so-called  precambrian  strata — have  been  de- 
posited in  equally  long  or  still  longer  periods,  we  see  that 
the  theory  of  Helmholtz  is  unsatisfactory. 

A  somewhat  peculiar  way  out  of  the  dilemma  has  been 
suggested  by  a  few  scientists.  We  know  that  one  gramme 
of  the  wonderful  element  radium  emits  about  120  calories 
per  hour,  or  in  the  course  of  a  year,  in  round  numbers, 
a  million  calories.  This  radiation  seems  to  continue  un- 
impaired for  years.  If  we- now  assume  that  each  kilo- 
gramme of  the  mass  of  the  sun  contains  only  two  milli- 
grammes of  radium,  that  amount  would  be  sufficient  to 
balance  the  heat  expenditure  of  the  sun  for  all  future 
ages.  Without  some  further  auxiliary  hypothesis,  we  / 
can,  however,  not  listen  to  this  suggestion.  It  prev 
supposes  that  heat  is  created  out  of  nothing.  Some 
scientists,  indeed,  believe  that  radium  may  absorb  a 
radiation,  coming  from  space,  in  some  unknown  manner 
and  convert  it  into  heat.  Before  we  enter  seriously  into 
a  discussion  of  this  explanation  we  shall  have  to  answer 
the  questions  where  that  radiation  comes  from  and  where 
it  takes  its  store  of  energy. 

We  must,  therefore,  again  search  for  another  source  of 
heat  energy  for  the  sun.  Before  we  can  hope  to  find  it, 
we  had  better  study  the  sun  itself  a  little. 

All  scientists  are  agreed  that  the  sun  is  of  the  same 
constitution  as  the  thousands  of  luminous  stars  which 
we  see  in  the  sky.  According  to  the  color  of  the  light 
which  they  emit,  stars  are  classified  as  white,  yellow, 
and  red  stars.  The  differences  in  their  light  become 

69 


WORLDS    IN    THE   MAKING 

much  more  distinct  when  we  examine  them  spectro 
scopically.  In  the  white  stars  the  helium  and  hydrogen 
lines  predominate  decidedly;  the  helium  stars  contain, 
in  addition,  oxygen.  Metals  are  comparatively  little  rep- 
resented; but  they  play  a  main  part  in  the  spectra  of 
the  yellow  stars,  in  which,  further,  some  bands  become 
visible.  In  the  spectra  of  the  red  stars  we  notice  many 
bands  which  indicate  that  chemical  compounds  are  pres- 
ent in  the  outer  portions.  Everybody  knows  that  the 
platinum  wire  or  the  filament  of  an  incandescent  lamp 
which  has  been  heated  to  incandescence  by  the  electric 
current  first  shines  reddish,  then  yellow  when  the  current 
is  increased,  and  finally  more  and  more  white.  At  the 
same  time  the  temperature  rises.  We  can  estimate  the 
temperature  from  the  brightness  of  the  glow.  If  we 
know  the  wave-length  of  the  radiations  of  that  color 
which  emits  the  greatest  amount  of  heat  in  the  spectrum 
(it  should  be  a  normal  spectrum),  it  is  easy  to  calculate 
the  temperature  of  the  star  from  Wien's  law  of  displace- 
ments. We  need  only  divide  2.89  by  the  respective 
wave-length  expressed  in  mm.  to  find  the  absolute  tem- 
perature of  the  star;  by  deducting  273  from  the  result, 
we  obtain  the  temperature  in  degrees  Cent,  on  the  ordi- 
nary scale.  For  the  sun  the  maximum  of  heat  radiation 
lies  near  wave-length  0.00055  (in  the  greenish-yellow 
light),  and  therefore  the  absolute  temperature  of  the 
radiating  disk  of  the  sun,  the  so-called  photosphere,  should 
be  5255°  absolute,  or  nearly  5000°  Cent.  But  our  atmos- 
phere weakens  the  sunlight,  and  it  also  causes  a  displace- 
ment of  the  maximum  radiation  in  the  spectrum.  The 
same  applies  to  the  sun's  own  atmosphere,  so  that  we 
have  to  adopt  a  higher  estimate  than  5000°  Cent.  By 
means  of  Stefan's  law  of  radiation,  the  solar  temperature 
has  been  estimated  at  about  6200°,  which  would  corre- 

70 


RADIATION  AND  CONSTITUTION   OF  THE  SUN 

spond  to  a  wave-length  of  about  0.00045  mm.  This  cor- 
rection is  therefore  significant.  About  half  of  it  has  to 
be  ascribed  to  the  influence  of  the  solar  atmosphere,  the 
other  half  to  the  terrestrial  atmosphere.  A  Hungarian 
astronomer,  Harkanyi,  has  determined  in  the  same  way 
the  temperature  of  several  white  stars  (Vega  and  Sirius), 
and  found  it  to  be  about  1000°  higher  than  that  of  the 
sun,  while  the  red  star  Betelgeuse,  the  most  prominent 
star  in  Orion,  would  have  a  temperature  by  2500°  lower 
than  that  of  the  sun. 

It  must  expressly  be  stated  that  in  making  these  esti- 
mates we  understand  by  the  temperature  of  the  star  in 
this  case  the  temperature  of  a  radiating  body  which  emits 
the  same  light  as  that  which  reaches  us  from  the  star. 
But  the  stellar  light  undergoes  important  changes  on  its 
way  to  us.  We  learn  from  observing  new  stars  t  that  a 
star  may  be  surrounded  by  a  cloud  of  cosmical  dust  which 
sifts  the  blue  rays  out  and  permits  the  red  ones  to  pass. 
The  star  then  shines  with  a  less  brilliantly  white  light  than 
in  the  absence  of  the  cloud.  The  consequence  is  that  we 
estimate  the  temperature  lower  than  it  really  is.  In  the 
red  stars  bands  have  been  noticed,  indicating,  as  we  have 
already  said,  the  presence  of  chemical  compounds.  The 
most  interesting  of  these  are  the  compounds  of  cyanogen 
and  of  carbon,  probably  with  hydrogen,  which  appear  to 
resemble  those  observed  by  Swan  in  the  spectrum  of  gas 
flames  and  which  were  named  after  him.  It  was  formerly 
thought  that  the  presence  of  these  compounds  implied 
lower  temperature.  But  we  shall  see  that  this  conclusion 
is  not  firmly  established.  Hale  has  found  during  eclipses 
of  the  sun  that  exactly  the  same  compounds  occur  im- 
mediately above  the  luminous  clouds  of  the  sun.  They 
are  probably  more  numerous  below  the  clouds,  where  the 
temperature  is  no  doubt  higher,  than  above  them. 

6  71 


WORLDS  IN  THE  MAKING 

However  that  may  be,  we  have  reason  to  assume  that 
the  now  yellow  sun  was  once  a  white  star  like  the  brill- 
iant Sirius,  that  it  has  slowly  cooled  down  to  its  present 
appearance,  and  that  it  will  some  day  shine  with  the  red- 
dish light  of  Betelgeuse.  The  sun  will  then  only  radiate 
a  seventh  of  the  heat  which  it  emits  now,  and  it  is  very 
likely  that  the  earth  will  have  been  transformed  into  a 
glacial  desert  long  before  that  time. 

It  has  already  been  pointed  out  that  the  atmospheres 
of  both  the  sun  and  of  the  earth  produce  a  strong  ab- 
sorption of  the  solar  rays,  and  especially  of  the  blue  and 
white  rays.  It  is  for  this  reason  that  the  light  of  the  sun 
appears  more  red  in  the  evening  than  at  noon,  because 
in  the  former  case  it  has  to  pass  through  a  thicker  layer 
of  air,  which  absorbs  the  blue  rays.  For  the  same  reason 
the  limb  of  the  sun  appears  more  red  in  spectroscopic 
examinations  than  the  centre  of  the  sun.  This  weaken- 
ing of  the  sun's  light  is  due  to  the  fine  dust  pervading 
the  atmospheres  of  the  earth  and  the  «un.  When  the 
products  of  strong  volcanic  eruptions,  like  the  eruptions 
of  Krakatoa  in  1883  and  of  Mont  Pelee  in  1902,  filled  the 
atmosphere  with  a  fine  volcanic  dust,  the  sun  appeared 
distinctly  red  when  standing  low  in  the  horizon.  It  was 
this  dust  that  caused  the  red  glow. 

When  we  examine  an  image  of  the  sun  which  has 
been  thrown  on  a  screen  by  the  aid  of  a  lens  or  a  system 
of  lenses,  we  notice  on  the  sun's  disk  a  mottling  of  charac- 
teristic darker  spots.  These  spots  struck  the  attention 
of  Galileo,  and  they  were  discovered  almost  simultane- 
ously by  him,  by  Fabricius,  and  by  Scheiner  (1610-1611). 
These  spots  have  since  been  the  most  diligently  studied 
features  of  the  sun.  We  carefully  determine  their  number 
and  sizes,  and  combine  these  two  data  to  make  the 
so-called  sun-spot  numbers.  These  numbers  change 

72 


RADIATION   AND   CONSTITUTION   OF   THE   SUN 

from  year  to  year  in  a  rather  irregular  way,  the  period 
amounting  on  an  average  to  11.1  years.  The  spots  appear 
in  two  belts  on  the  sun,  and  they  glide  over  the  disk  in 
the  course  of  thirteen  or  fourteen  days.  Sometimes  they 
reappear  after  another  thirteen  or  fourteen  days.  It  is 
therefore  believed  that  they  lie  comparatively  quiet  on  the 
surface  of  the  sun,  and  that  the  sun  rotates  about  its  own 
axis  in  about  twenty-seven  days,  so  that  after  that  period 
the  same  points  are  again  opposite  the  earth.  This  is 
the  so-called  synodical  period.  The  great  interest  which 
attaches  to  the  study  of  these  features  lies  in  the  fact  that 
simultaneously  with  these  spots  several  other  phenomena 
seem  to  vary  which  attain  their  maxima  at  the  same  time. 
Such  are,  in  the  first  instance,  the  polar  lights  and  the 
magnetic  variations,  and,  to  a  lesser  degree,  the  cirrus 
clouds  and  temperature  changes,  as  well  as  several  other 
meteorological  phenomena  (compare  Chapter  V.). 

About  the  sun-spots  we  notice  the  so-called  faculse — 
portions  which  are  much  brighter  than  their  surround- 
ings. When  we  carefully  examine  a  strongly  magnified 
image  of  the  sun,  we  find  that  it  has  a  granulated  ap- 
pearance (Fig.  18).  Langley  compares  the  disk  to  a 
grayish-white  cloth  almost  hidden  by  flakes  of  snow. 
The  less  bright  portions  are  designated  "pores,"  the 
brighter  portions  "granules."  It  is  generally  assumed 
that  the  granules  correspond  to  clouds  which  rise  like 
the  clouds  of  our  atmosphere  on  the  top  of  ascending 
convection  currents.  But  while  the  terrestrial  clouds  are 
formed  of  drops  of  rain  or  of  crystals  of  ice,  the  granules 
consist  probably  of  soot — that  is  to  say,  condensed  car- 
bon— and  of  drops  of  metals,  iron,  and  others.  The 
smallest  granule  which  we  are  able  to  discern  has  a 
diameter  of  about  200  km.  (130  miles). 

The  faculse  are  formed  by  very  large  accumulations  of 

73 


WORLD^   IN    THE   MAKING 


Fig.  18. — Sun-spot  group  and  granulation  of  the  sun.    (Photographed 
at  the  Meudon  Observatory,  near  Paris,  April  1,  1884) 

clouds  which  are  carried  up  by  strong  ascending  currents 
and  spread  over  large  areas,  as  in  our  cyclones.  The 
spots  correspond  to  descending  masses  of  gas  with  rising 
temperatures,  which  are  therefore  "dry"  and  do  not 
carry  any  clouds,  as  in  terrestrial  anticyclones.  Through 

74 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

these  holes  in  the  walls  of  solar  clouds  we  peep  a  little 
farther  into  the  gigantic  masses  of  gas,  and  we  obtain  an 
idea  of  the  state  of  affairs  in  the  deeper  strata  of  the  sun. 
The  depth  of  the  wall  of  cloud  is,  of  course,  not  large 
compared  to  the  radius  of  the  sun. 

The  study  of  the  spectra  affords  us  the  best  insight 
into  the  nature  of  the  different  parts  of  the  sun.  The 
spectra  teach  us  not  only  the  constituents  of  these  parts, 
but  also  the  velocities  with  which  they  move.  We  have 
learned  in  this  way  that,  lying  above  the  luminous  clouds 
of  the  sun  which  are  radiating  to  us,  there  are  great  masses 
of  gas  containing  most  of  our  terrestrial  elements.  We 
distinguish  particularly  in  them  iron,  magnesium,  cal- 
cium, sodium,  helium,  and  hydrogen.  The  two  last- 
mentioned  constituents,  being  the  least  dense,  are  found 
particularly  in  the  outermost  strata  of  the  atmosphere. 
The  solar  atmosphere  becomes  visible  when,  during  an 
eclipse  of  the  sun,  the  disk  of  the  moon  has  proceeded  so 
far  as  to  cover  the  intensely  luminous  clouds  in  the  so- 
called  photosphere.  Owing  to  its  strong  percentage  of 


Fig.  19. —Part  of  the  solar  spectrum  of  January  3, 1872.  After  Langley. 
The  bright  horizontal  bands  are  due  to  prominences.  .  In  the  middle 
(at  208)  the  hydrogen  line  F,  strongly  distorted  by  violent  agitation 

75 


WORLDS    IN    THE    MAKING 

hydrogen,  the  gaseous  atmosphere  generally  shines  in  the 
purple  hue  which  is  characteristic  of  this  element.  This 
stratum  of  gas  is  also  called  the  chromosphere  (from  the 
Greek  word  %p<w//,a,  meaning  color).  Its  thickness  is 
estimated  at  from  7000  to  9000  km.  (5000  to  6000  miles). 


Fig.    20.  —  Metallic    promi- 
nences in  vortex   motion 

The  white  spot  marks  the  Fig.    21. — Fountain-like   metallic 
size  of  the  earth  prominences 

From  it  rise  rays  of  fire  over  the  surrounding  surface  like 
blades  of  grass  on  meadows,  to  which  their  appearance 
has  been  likened. 

When  these  flames  rise  still  higher,  to  about  15,000 
km.  (9300  miles)  or  more,  they  are  called  protuberances 
or  prominences.  Their  number  as  well  as  their  altitude 
grow  with  the  number  of  sun  -  spots.  They  are  dis- 
tinguished as  metallic  and  as  quiet  prominences.  The 
former  are  characterized  by  particularly  violent  mo- 
tion, as  will  become  apparent  from  Figs.  20  and  21, 
and  they  contain  large  amounts  of  metallic  vapor. 
They  appear  only  within  the  belt  of  sun-spots  which 
are  most  pronounced  at  a  distance  of  about  20°  from 
the  solar  equator.  Their  movements  are  so  violent  that 
they  often  traverse  several  hundreds  of  kilometres  in  a 
second.  The  Hungarian  Fenyi  observed,  indeed,  on  July 
15, 1895,  a  prominence  whose  greatest  velocity  in  the  line 

76 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

of  sight,  measured  spectroscopically,  amounted  to  862  km. 
(536  miles) ,  and  whose  maximum  velocity  at  right  angles 
to  this  direction  was  840  km.  per  second.  These  colossal 
velocities  distinguish  the  highest  parts,  while  the  lower 
portions,  which  are  the  most  dense  and  which  contain 
most  metallic  vapor,  are  less  mobile,  as  might  be  expected. 
Their  altitude  above  the  sun's  surface  may  reach  ex- 
ceedingly high  figures,  and  this  applies  also  to  the  quiet 
prominences.  The  above-mentioned  prominence  of  July 
15,  1895,  reached  a  height  of  500,000  km.,  and  Langley 
observed,  on  October  7, 1880,  one  at  an  altitude  of  560,000 
km.,  whose  tip,  therefore,  nearly  attained  an  elevation 
equal  to  that  of  a  radius  of  the  sun,  690,000  km.  above 
the  limb  of  the  sun's  photosphere.  The  mean  altitude 
of  these  prominences  is  40,000  km.  After  their  dis- 
covery by  Lector  Vassenius,  of  Gotheborg,  in  1733,  they 
could  only  be  studied  during  total  solar  eclipses,  until 


Fig;.  23. — Quiet  prominences,  shape 
Fis;.    22. — Quiet     prominences     of  of  a  tree.  ,  The  white  spot  indi- 

smoke-column  type  cates  the  size  of  the  earth 

Lockyer  and  Janssen  taught  us,  in  the  year  1868,  how 
to  observe  them  in  full  sunlight  by  means  of  the  spec- 
troscope. 
The  quiet  prominences  consist  almost  exclusively  of 

77 


WORLDS    IN    THE   MAKING 


hydrogen  and  helium;  sometimes  they  contain  also 
traces  of  metallic  gases.  They  resemble  clouds  floating 
quietly  in  the  solar  atmosphere,  or  masses  of  smoke  com- 


if 


Fig.  24. — Diagram  illustrating  the  differences  in  the  spectra  of  sun- 
spots  and  of  the  photosphere.  Some  lines  in  the  spot  spectrum  are 
stronger,  others  fainter,  than  in  the  photosphere  spectrum.  In  the 
central  portion,  two  reversals;  to  the  right,  two  bands.  After  Mitchell 

ing  from  a  chimney.  They  may  appear  anywhere  on  the 
sun,  and  their  stability  is  so  great  that  they  have  some- 
times been  watched  during  a  complete  solar  rotation  (for 


Fig.  25. — Spectrum  of  a  sun-spot,  the  central  band  between  the  two 
portions  of  the  photosphere  spectrum.  The  spot  spectrum  is  bor- 
dered with  the  half -shadows  of  the  edge  of  the  spot.  After  Mitchell 

78 


RADIATION   AND  CONSTITUTION  OF   THE   SUN 

about  forty  days) ;    this  is  possible  only  when  they  occur 
in  the  neighborhood  of  the  poles,  where  they  always  re- 
main visible   outside   the  sun's  limb.     Figs.  22  and  23 
show  several  such  prominences  according  to  Young. 
Sometimes  the  matter  of  the  prominences  seems  to  fall 


A7 


E 


V 


w 


Fig.  26. — The  great  sun-spot  of  October  9,  1903.  Taken  with  the 
photo-heliograph  of  Greenwich  in  the  usual  manner.  The  spot  is 
shown  at  mean  level  of  the  calcium  faculse.  The  two  following 
photographs  show  a  lower-level  and  a  higher-level  section  through 
the  calcium  faculae 

back  upon  the  surface  of  the  sun  between  the  smaller 
flames  of  fire  which  we  have  likened  to  blades  of  grass 
(Fig.  21).  In  most  cases,  however,  the  prominences  ap- 
pear slowly  to  dissolve.  When  their  brilliant  glow  fades 
owing  to  their  intense  radiation,  they  can  no  longer 

79 


WORLDS    IN    THE   MAKING 

be  observed.  The  quiet  prominences,  which  seem  to 
float  at  heights  of  about  50,000  km.  and  at  still  greater 
heights,  must  there  be  almost  in  a  vacuum.  Their  par- 
ticles cannot  be  supported  by  any  surrounding  gases, 
after  the  manner  of  the  drops  of  water  in  terrestrial 
clouds.  In  order  that  they  may  remain  floating  they 
must  be  pushed  away  from  the  sun  by  a  peculiar  force — 
the  radiation  pressure  (see  Chapter  IV.). 

The  facula3  can  be  studied  in  the  same  way  as  the 
prominences,  and  of  late  Deslandres  and  Hale  have  used 
for  this  purpose  a  special  instrument,  the  heliograph 
(compare  Figs.  26  to  29).  When  the  faculse  approach 
the  limb  of  the  sun  they  appear  particularly  brilliant  by 
comparison  with  their  surroundings.  That  seems  to  in- 
dicate that  they  are  lying  at  a  great  altitude.,  and  that 


Fig.  27. — The  great  sun-spot  of  October  9, 1903.  Photograph  of  the 
low-level  calcium  faculae  with  the  aid  of  the  light  of  the  calcium 
line  H.  The  spot  is  not  obscured  by  the  faculan — at  least,  not  so 
much  as  in  the  following  illustrations 

SO 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

their  light  is  hence  not  weakened  by  the  superposed  hazy 
stratum.  When  they  reach  the  sun's  limb  they  appear 
to  us  like  raised  portions  of  the  photosphere.  The  clouds 


Fig.  28.— The  great  sun-spot  of  October  9,  1903.  Photograph  of  the 
higher-level  calcium  faculse,  taken  with  the  light  of  the  central  portion 
of  the  line  H  (calcium).  The  higher-level  faculse  hide  the  spot,  in- 
dicating that  the  faculaB  spread  considerably  during  their  ascent 


which  form  these  faculse  are  carried  upward  by  powerful 
ascending  streams  of  gas  whose  expansion  is  due  to  the 
diminution  of  the  gaseous  pressure. 

Sun-spots  display  many  peculiarities  in  their  spectra 
(Figs.  24  and  25).  Very  prominent  is  always  the  helium 
line ;  prominent  likewise  the  dark  sodium  lines,  which  are 
markedly  widened  and  which  show  in  their  middle  portions 
a  bright  line — the  so-called  reversal  of  lines  (Fig.  24) .  This 
occurrence  indicates  that  the  metal  is  lying  in  a  deeper 
stratum.  In  the  red  portion  of  the  spectrum  we  find 

81 


WORLDS    IN    THE    MAKING 

bands,  just  as  in  the  spectra  of  the  red  stars.  These  bands, 
which  appear  to  be  resolved  into  crowds  of  lines  by  the 
aid  of  powerful  instruments,  indicate  the  presence  of 
chemical  compounds.  Since  the  spot  is  comparatively 
of  feeble  intensity,  its  spectrum  appears  superposed 
like  a  less  bright  ribbon  upon  the  background  of  the 
spectrum  of  the  more  luminous  photosphere.  The  violet 
end  of  the  sun-spot  spectrum  is  particularly  weakened. 
Although  the  spot  has  the  appearance  of  a  pit  in  the 


Fig.  29. — The  great  sun-spot  of  October  9,  1903.  Photograph  of  the 
hydrogen  faculse,  taken  with  the  light  of  the  spectral  line  F  (hydro- 
gen). Only  the  darkest  portions  of  the  spot  are  visible.  The  other 
portions  are  obscured  by  masses  of  the  hydrogen,  which  were  evidently 
in  a  restless  state 


photosphere,  and  when  on  the  sun's  limb  makes  it  look 
as  if  a  piece  had  been  cut  out  of  the  edge,  it  yet 
does  not  appear  darker  than,  the  sun's  edge.  That  points 

82 


RADIATION   AND  CONSTITUTION   OF  THE  SUN 


Fig.  30. — Photograph  of  the  solar  corona  of  1900,  (After 
Langley  and  Abbot.)  Illustrating  the  appearance  of  the 
corona  in  years  of  minimum  sun-spot  frequency 

to  the  conclusion  that  the  light  emitted  by  the  spot 
emanates  chiefly  from  its  upper,  cold  portions. 

The  light  coming  from  the  deeper  portions  is  distinctly 
absorbed  to  a  large  degree  by  the  higher-lying  strata. 
The  sun-spots  also  appear  to  become  narrower  in  their 
lower  parts,  owing  to  the  compression  of  the  gases  at 
greater  depths,  and  one  may  regard  their  funnel-shaped 
cloud-walls  as  "  half-shadows,"  which  appear  darker  than 
the  surroundings,  but  brighter  than  the  so-called  core  of 
the  spot.  The  weakening  of  the  violet  end  of  the  spec- 
trum is  probably  due  to  the  presence  of  fine  particles  of 

83 


WORLDS    IN    THE    MAKING 

dust  in  the  solar  gases,  just  as  they  cause*  the  corre- 
sponding weakening  of  the  violet  end  of  the  spectrum  of 
the  sun's  limb.  The  bands  in  the  red  parts  of  the  sun- 
spot  spectrum  may  originate  from  the  deeper  portions 
of  the  spot,  because  all  the  higher  parts  of  the  solar 


Fig.  31. — Photograph  of  the    solar  corona  of  1870.     (After  Davis.) 
The  year  1870  was  one  of  maximum  sun-spot  frequency 


atmosphere  yield  simple,  sharp  lines.  The  bands  suggest 
that  chemical  compounds  can  exist  at  the  higher  press- 
ure of  the  inner  portions  of  the  sun,  and  that  these  com- 

84 


RADIATION  AND  CONSTITUTION  OF  THE  SUN 

pounds  are  decomposed  in  the  outer  parts  of  the  sun,  to 
give  the  line  spectra  of  chemical  elements. 

The  enigmatical  corona  lies  farther  out  in  the  atmos- 
phere of  the  sun.  It  consists  of  streamers  which  may 
extend  beyond  the  disk  of  the  sun  to  the  length  of  several 
solar  diameters.  The  corona  can  only  be  observed  at 
total  eclipses  of  the  sun.  Figs.  30  to  32  illustrate  the  ap- 
pearance of  this  very  peculiar  phenomenon. 

When  the  number  of  sun-spots  is  small,  the  corona 
streamers  extend  like  huge  brooms  from  the  equatorial 


Fig.  32. — Photograph  of  the  solar  corona  of  1898.    (After  Maunder.) 
1898  was  a  year  of  average  solar  activity 

parts,  and  the  feebler  rays  of  the  corona  near  the  solar 
poles  are  then  bent  downward  to  the  equator,  just  like' 
the  lines  of  force  about  the  poles  of  a  magnet  (Fig.  30). 
We  suppose,  for  this  reason,  that  the  sun  acts  like  a 
strong  magnet,  whose  poles  are  situated  near  the  geo- 
graphical poles  of  the  sun.  In  years  which  are  richer  in 
sun-spots  the  distribution  of  the  streamers  of  the  corona 
is  more  uniform.  At  moderate  sun-spot  frequency,  large 
numbers  of  rays  seem  to  emanate  from  the  neighborhood 

85 


WORLDS    IN   THE   MAKING 

of  the  maximum  belt  of  sun-spots,  so  that  the  corona 
often  assumes  a  quadrangular  shape  (compare  Fig.  32). 

These  remarks  hold  for  the  "outer  corona,"  while  the 
inner  portion,  the  so-called  "inner  corona,"  shines  in  a 
more  uniform  light.  The  spectroscopic  examination 
demonstrates  that  the  light  consists  mainly  of  hydrogen 
gas  and  of  an  unknown  gas  designated  coronium,  which 
particularly  seems  to  occur  in  the  higher  parts  of  the 
inner  corona.  The  outer  streamers  of  the  corona,  on  the 
contrary,  yield  a  continuous  spectrum  which  shows  that 
the  light  is  radiated  by  solid  or  liquid  particles.  In  the 
spectrum  of  the  coronal  rays  at  an  extreme  distance  from 
the  disk,  astronomers  have  sometimes  fancied  that  they 
discerned  dark  lines  on  a  bright  ground,  just  as  in  the 
spectrum  of  the  photosphere.  It  has  been  assumed  that 
this  light  is  reflected  sunlight,  originating  from  small  solid 
or  liquid  particles  of  the  outer  corona.  It  must  be  re- 
flected, because  it  is  partly  polarized.  The  radiating  dis- 
position of  the  outer  corona  indicates  the  action  of  a 
force,  the  radiation  pressure,  which  drives  the  smaller 
particles  away  from  the  centre  of  the  sun. 

As  regards  the  temperature  of  the  sun,  we  have  already 
seen  that  the  two  methods  applied  for  its  determination 
have  yielded  somewhat  unequal  results.  From  the  in- 
tensity of  the  radiation,  Christiansen,  and  afterwards 
Warburg,  calculated  a  temperature  of  about  6000°  Cent. 
Wilson  and  Gray  found  for  the  centre  of  the  sun  6200°, 
which  they  afterwards  corrected  into  8000°.  Owing  to 
the  absorption  of  light  by  the  terrestrial  and  the  solar 
atmospheres,  we  always  find  too  low  values.  That  ap- 
plies, to  a  still  greater  extent,  to  any  estimate  based  upon 
the  determination  of  that  wave-length  for  which  the  heat 
emission  from  the  solar  spectrum  is  maximum.  Lc 
Chatelier  compared  the  intensity  of  sunlight  filtered 

86 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

through  red  glass  with  the  intensities  of  light  from  sev- 
eral terrestrial  sources  of  fairly  well-known  temperatures 
treated  in  the  same  way.  These  estimates  yielded  to 
him  a  solar  temperature  of  7600°  Cent.  Most  scientists 
reckon  with  an  absolute  temperature  of  6500°,  corre- 
sponding to  about  6200°  Celsius.  That  is  what  is  known 
as  the  "effective  temperature"  of  the  sun.  If  the  solar 
rays  were  not  partially  absorbed,  this  temperature  would 
correspond  to  that  of  the  clouds  of  the  photosphere.  Since 
red  light  is  little  absorbed  comparatively,  Le  Chatelier's 
value  of  7600°,  and  the  almost  equal  value  of  Wilson  and 
Gray  of  8000°,  should  approximately  represent  the  average 
temperature  of  the  outer  portions  of  the  clouds  of  the 
photosphere.  The  higher  temperature  of  the  faculse  is 
evident  from  their  greater  light  intensity,  which,  how- 
ever, may  partly  be  due  to  their  greater  height.  Carring- 
ton  and  Hodgson  saw,  on  September  1,  1859,  two  faculse 
break  out  from  the  edge  of  a  sun-spot.  Their  splendor 
was  five  or  six  times  greater  than  that  of  the  surrounding 
parts  of  the  photosphere.  That  would  correspond  to 
a  temperature  of  about  10,000  or  12,000°  Cent:  The 
deeper  parts  of  the  sun  which  broke  out  on  these  occa- 
sions evidently  have  a  higher  temperature,  and  this  is 
not  unnatural,  since  the  sun  is  losing  heat  by  radiation 
from  its  outer  portions. 

We  know  that  the  temperature  of  our  atmosphere  de- 
creases with  greater  heights.  The  movements  of  the  air 
are  concerned  in  this  change.  A  sinking  mass  of  air  is 
compressed  by  the  increased  pressure  to  which  it  is  being 
exposed,  and  its  temperature  rises,  therefore,  just  as  the 
temperature  rises  in  a  pneumatic  gas-lighter  when  the 
piston  is  pressed  down.  If  the  air  were  dry  and  in  strong 
vertical  motion,  its  temperature  would  change  by  10°  Cent. 
(18°  F.)  per  km.  If  it  stood  still,  it  would  assume  an  al- 
7  87 


WORLDS   IN   THE   MAKING 

most  uniform  temperature;  that  is  to  say,  there  would 
be  no  lowering  of  the  temperature  as  we  proceed  upward. 
The  actual  value  lies  between  the  two  extremes.  As  the 
gravitation  in  the  photosphere  of  the  sun  is  27.4  times 
greater  than  on  the  surface  of  the  earth,  we  can  deduce 
that,  if  the  air  on  the  sun  were  as  dense  as  on  the  earth, 
the  temperature  on  the  sun  would  vary  27.4  times  as 
much  as  on  the  earth  with  the  increasing  height — that 
is  to  say,  by  270  degrees  per  kilometre,  provided  its  atmos- 
phere were  in  violent  agitation.  Now,  the  outer  portions 
of  the  solar  atmosphere  are,  indeed,  in  violent  motion,  so 
that  this  latter  assumption  seems  to  be  justified.  But 
this  part  consists  essentially  of  hydrogen,  which  is  29 
times  lighter  than  the  air.  We  must,  therefore,  reduce 
the  value  at  which  we  arrive  to  one-twenty-ninth.  As  a 
result,  the  final  temperature  gradient  per  kilometre  would 
only  be  9°  Cent.  (16.2°  F.).  But  the  radiation  is  extreme- 
ly powerful  on  the  sun,  and  it  tends  to  equalize  the  con- 
ditions. Nine  degrees  per  kilometre  is  therefore,  without 
doubt,  too  high  a  value.  Further,  in  the  interior  of  the 
sun  the  gases  are  much  heavier.  At  a  small  depth,  how- 
ever, they  will  be  so  strongly  compressed  by  the  upper 
strata  that  their  further  compressibility  will  be  limited, 
and  the  calculation  which  we  have  just  made  loses  its 
validity.  Yet,  in  any  case,  the  temperature  of  the  sun 
must  increase  as  we  penetrate  nearer  to  its  centre.  If 
we  accept  a  temperature  gradient  per  kilometre  of  the 
value  above  indicated,  9° — it  is  three  times  greater  in  the 
solid  earth-crust — we  should  obtain  for  the  centre  of  the 
sun  a  temperature  of  more  than  six  million  degrees. 

All  substances  melt  and  evaporate  as  their  tempera- 
ture is  raised.  If  the  temperature  exceeds  a  certain 
limit,  the  "critical  temperature,"  the  substance  can  no 
longer  be  condensed  to  a  liquid,  however  high  the  press- 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

ure  may  be  pushed,  and  the  substance  will  only  exist 
as  a  gas.  If  we  start  from  — 273°  as  absolute  zero, 
this  critical  temperature  is  nearly  one  and  a  half  times 
as  high  as  the  ebullition  temperature  of  the  sub- 
stance under  atmospheric  pressure.  So  far  as  our  ex- 
perience goes,  it  does  not  appear  probable  that  the 
critical  temperature  of  any  substance  could  be  higher 
than  10,000°  or  12,000°  Cent.,  the  highest  values  which 
we  have  calculated  for  the  temperature  of  the  faculae. 
The  inner  portions  of  the  sun  must  hence  be  gaseous, 
and  the  whole  sun  be  a  strongly  compressed  mass  of  gas 
of  extremely  high  temperature,  which,  owing  to  the  high 
pressure,  is  at  a  density  1.4  times  as  great  as  that  of 
water,  and  which  in  many  respects,  therefore,  will  re- 
semble a  liquid.  It  must,  for  instance,  be  extremely 
viscid,  and  that  accounts  for  the  relatively  great  stability 
of  the  sun-spots  (one  sun-spot  held  out  for  a  year  and  a 
half  in  1840  and  1841).  The  sun  would  thus  have  to  be 
regarded  as  a  sphere  of  gas,  in  the  outer  portions  of  which 
a  certain  amount  of  condensations  of  cloud  character 
have  taken  place,  owing  to  radiation  and  to  the  out- 
ward movements  of  the  gaseous  masses.  The  pressure 
in  the  photosphere  —  that  is,  in  those  parts  in  which 
these  clouds  are  floating — has  been  averaged  at  five  or 
six  atmospheres,  a  figure  which,  considering  the  very 
high  gravitation,  would  suggest  a  layer  of  superposed 
gas  above  it  corresponding  to  not  more  than  a  fifth 
of  our  terrestrial  atmosphere.  At  an  approximately  cor- 
responding height,  11,500  m.  (38,000  ft.),  there  are  float- 
ing in  the  terrestrial  atmosphere  the  highest  cirrus  clouds, 
to  which  the  clouds  of  the  photosphere  may  in  many 
respects  be  compared. 

We  turn  back  to  the  unanswered  question  whence  the 
sun  takes  the  compensation  for  the  heat  which  it  con- 

89 


WORLDS    IN    THE   MAKING 

stantly  radiates  into  space.  The  most  powerful  source 
of  heat  known  to  us  is  that  of  chemical  reactions.  The 
most  familiar  reaction  of  daily  life  is  the  combustion  of 
coal.  By  burning  one  gramme  of  carbon  we  obtain 
8000  calories.  If  the  sun  consisted  of  pure  carbon,  its 
energy  would  not  hold  out  more  than  4000  years.  It  is 
not  to  be  wondered  at,  therefore,  that  most  scientists 
soon  abandoned  the  hope  of  solving  the  problem  in  this 
way.  The  French  astronomer  Faye  attempted  to  explain 
the  replenishment  of  the  losses  of  heat  by  radiation 
from  the  sun  by  arguments  in  which  he  resorted  to 
the  heat  of  a  combination  of  the  constituents  of  the 
sun.  He  said:  "So  high  a  temperature  must  prevail  in 
the  interior  of  the  sun  that  everything  there  will  be  de- 
composed into  its  elementary  constituents.  When  the 
atoms  afterwards  penetrate  into  the  outer  layers,  they 
are  again  united,  and  they  liberate  heat."  Faye  thus 
imagined  that  new  masses  of  elements  would  constantly 
rise  from  the  interior  of  the  sun  and  would  be  reunited 
in  chemical  combination  on  the  surface.  But  if  new 
masses  are  to  penetrate  upward  to  the  surface,  those 
which  were  at  first  above  must  go  back  to  the  centre  of 
the  sun,  in  order  to  be  re-decomposed  by  the  great  heat 
there;  and  this  re-decomposition  would  consume  just  as 
much  heat  as  was  gained  by  the  rising  of  the  same  masses 
to  the  surface.  This  convection  can  therefore  only  help 
to  transport  the  store  of  heat  from  the  interior  to  the 
surface.  The  total  amount  of  heat  stored  in  the  sun 
would  in  this  way,  supposing  the  mean  temperature  to 
be  six  million  degrees,  be  able  to  cover  the  heat  ex- 
penditure for  about  three  million  years. 

We  have,  moreover,  seen  that  the  highest  strata 
of  the  sun  are  distinguished  by  line  spectra,  suggestive 
of  simple  chemical  compounds,  while  at  greater  depth 

90 


RADIATION   AND   CONSTITUTION   OF  THE   SUN 

in  the  sun-spots  chemical  combinations  occur  which 
are  characterized  by  band  spectra.  It  is  quite  incorrect 
to  assert  that  high  temperatures  must  necessarily  de- 
compose all  chemical  compounds  into  their  elements. 
The  mechanical  theory  of  heat  teaches  us  only  that  at 
rising  temperatures  products  are  formed  whose  formation 
goes  hand  in  hand  with  an  absorption  of  heat.  Thus,  at 
a  high  temperature,  ozone  is  formed  from  oxygen,  al- 
though ozone  is  more  complex  in  composition  than 
oxygen,  and  by  this  reaction  750  calories  are  consumed 
when  one  gramme  of  oxygen  is  transformed  into  one 
gramme  of  ozone.  We  likewise  know  that  in  the  electric 
arc,  at  a  temperature  of  about  3000°,  a  compound  is 
formed  under  consumption  of  heat  by  the  oxygen  and 
nitrogen  of  the  atmosphere.  A  new  method  for  the  tech- 
nical preparation  of  nitric  acid  from  the  nitrogen  of  the 
air  is  based  upon  this  reaction.  Again,  the  well-known 
compounds  benzene  and  acetylene  are  formed  from  their 
elements,  carbon  and  hydrogen,  under  absorption  of 
heat.  All  these  bodies  can  only  be  synthetized  from  their 
elementary  constituents  at  high  temperatures.  We  fur- 
ther know  from  experience  that  the  higher  the  temper- 
ature at  which  a  reaction  takes  place,  the  greater,  in 
general,  the  amount  of  heat  which  it  absorbs. 

A  similar  law  applies  to  the  influence  of  pressure. 
When  the  pressure  is  increased,  such  processes  will  be 
favored  as  will  yield  products  of  a  smaller  volume.  If 
we  imagine  that  a  mass  of  gas  rushes  down  from  a  higher 
stratum  of  the  sun  into  the  depths  of  the  sun's  in- 
terior, as  gases  do  in  sun-spots,  complex  compounds  will 
be  produced  by  virtue  of  the  increased  pressure.  This 
pressure  must  increase  at  an  immense  rate  towards  the 
interior  of  the  sun,  by  about  3500  atmospheres  per  kilo- 
metre. The  gases  which  dissociate  into  atoms  at  the  lower 

91 


WORLDS    IN   THE   MAKING 

pressures  and  the  higher  temperatures  of  the  extreme 
solar  strata  above  the  photosphere  clouds  enter  into 
chemical  combination  in  the  depths  of  the  spots,  as  we 
learn  from  spectroscopic  examination.  Owing  to  their 
high  temperatures,  these  compounds  absorb  enormous 
quantities  of  heat  in  their  building  up,  and  these  quanti- 
ties of  heat  are  to  those  which  are  concerned  in  the  chemi- 
cal processes  of  the  earth  in  the  same  ratio  as  the  tem- 
perature of  the  sun  is  to  that  at  which  the  chemical 
reactions  are  proceeding  on  the  earth.  As  these  gases 
penetrate  farther  into  the  sun,  temperature  and  pressure 
are  still  more  and  more  increased,  and  there  will  result 
products  more  and  more  abounding  in  energy  and  con- 
centration. We  may,  therefore,  imagine  the  interior  of  the 
sun  charged  with  compounds  which,  brought  to  the  sur- 
face of  the  sun,  would  dissociate  under  an  enormous  evolu- 
tion of  heat  and  an  enormous  increase  of  volume.  These 
compounds  have  to  be  regarded  as  the  most  powerful 
blasting  agents,  by  comparison  with  which  dynamite  and 
gun-cotton  would  appear  like  toys.  In  confirmation  of 
this  view,  we  observe  that  gases  when  penetrating  into 
the  photosphere  clouds  are  able  to  eject  prominences  at 
a  stupendous  velocity,  attaining  several  hundred  kilo- 
metres per  second.  This  velocity  surpasses  that  of  the 
swiftest  rifle-bullet  about  a  thousandfold.  We  may  hence 
ascribe  to  the  explosives  which  are  confined  in  the  in- 
terior of  the  sun  energies  which  must  be  a  million  times 
greater  than  the  energy  of  our  blasting  agents.  (For  the 
energy  increases  with  the  square  of  the  velocity.)  And 
yet  these  solar  blasting  agents  have  already  given  up 
a  large  part  of  their  energy  during  their  passage  from  the 
sun's  interior.  It  thus  becomes  conceivable  that  the 
solar  energy — instead  of  holding  out  for  4000  years,  as 

it  would  if  it  depended  upon  the  combustion  of  a  solar 

92 


RADIATION   AND   CONSTITUTION  OF  THE   SUN 

sphere  made  out  of  carbon — will  last  for  something  like 
four  thousand  million  years.  Perhaps  we  may  further 
extend  this  period  to  several  billions. 

That  there  are  such  energetic  compounds  we  have 
learned  from  the  discovery  of  the  heat  evolution  of  radium. 
According  to  Rutherford,  radmn^is  decomposed  by  one- 
half  in  the  space  of  about  W&&  years.  In  this  decom- 
position a  quantity  of  about  a  million  calories  is  evolved 
per  gramme  and  per  year,  and  we  thus  find  that  the  de- 
composition of  radium  into  its  final  products  is  accom- 
panied by  a  heat  evolution  of  about  two  thousand  millions 
of  calories  per  gramme — about  a  quarter  of  a  million 
times  more  heat  than  the  combustion  of  one  gramme  of 
carbon  would  yield. 

In  chemical  respects  as  well,  then,  the  earth  is  a  dwarf 
compared  to  the  sun,  and  we  have  every  reason  to  pre- 
sume that  the  chemical  energy  of  the  sun  will  be  sufficient 
to  sustain  the  solar  heat  during  many  thousand  millions 
and  possibly  billions  of  years  to  come. 


IV 

THE    RADIATION   PRESSURE 

NEXT  to  simple  measuring  and  simple  calculations, 
astronomy  appears  to  be  the  most  ancient  science.  Yet, 
though  man  has  worshipped  the  sun  from  the  most  remote 
ages,  it  was  not  fully  comprehended  before  the  middle 
of  the  past  century  that  the  sun  is  the  source  of  all 
life  and  of  all  motion.  Part  of  the  veneration  for  the 
sun  was  transferred  to  the  moon,  with  its  mild  light,  and 
to  the  smaller  celestial  lights.  It  did  not  escape  notice 
that  their  positions  in  the  sky  were  always  changing 
simultaneously  with  the  annual  variations  in  the  weather, 
and  all  human  undertakings  depended  upon  the  weather 
and  the  seasons.  The  moon  and  the  stars  were  worship- 
ped— we  know  now,  without  any  justification  whatever — 
as  ruling  over  the  weather,  and  consequently  over  man's 
fate.1  Before  anything  was  undertaken  people  attempt- 
ed first  to  assure  themselves  of  the  favorable  aspect  of 
the  constellations,  and  since  the  most  remote  ages  as- 
trologers have  exercised  a  vast  influence  over  the  igno- 
rant and  superstitious  multitude. 

In  spite   of   the  vehement  enunciation   of   Giordano 

1  The  moon  strongly,  and  more  than  any  other  agent,  influences 
the  tides.  Apart  from  this  effect  the  position  of  the  moon  has  only 
a  feeble  influence  upon  the  air  pressure  and  upon  atmospheric  elec- 
tricity and  terrestrial  magnetism.  The  influence  of  the  stars  is  im- 
perceptible. 

94 


THE   RADIATION   PRESSURE 

Bruno  (1548-1600),  this  superstition  was  still  deeply 
rooted  when  Newton  succeeded  in  proving,  in  1686, 
that  the  movements  of  the  so-called  wandering  stars, 
or  planets,  and  of  their  moons  could  be  calculated 
with  the  aid  of  one  very  simple  law:  that  all  these 
celestial  bodies  are  attracted  by  the  sun  or  by  their 
respective  central  bodies  with  a  force  which  is  pro- 
portional to  their  own  mass  and  to  the  mass  of  the 
central  body  and  inversely  proportional  to  the  square 
of  their  distance  from  that  central  body.  Newton's 
contemporary,  Halley,  applied  the  law  of  gravitation 
also  to  the  mysterious  comets,  and  calculating  astron- 
omy has  since  been  based  upon  this,  its  firmest  law, 
to  which  there  has  not  been  found  any  exception.  The 
world  was  thus  at  once  rid  of  the  paralyzing  superstition 
which  exacted  belief  in  a  mysterious  ruling  of  the  stars. 
The  contemporaries  of  Newton,  as  well  as  their  descend- 
ants, have  rightly  valued  this  discovery  more  highly 
than  any  other  scientific  triumph  of  this  hero's.  Ac- 
cording to  Newton's  law,  all  material  bodies  would  tend 
to  become  more  and  more  concentrated  and  united,  and 
the  development  of  the  universe  would  result  in  the  suck- 
ing up  of  the  smaller  celestial  bodies — the  meteorites,  for 
instance — by  the  larger  bodies. 

It  must,  however,  be  remarked  that  Newton's  great 
precursor,  Kepler,  observed  in  1618  that  the  matter  of 
the  comets  is  repelled  by  the  sun.  Like  Newton,  he 
believed  in  the  corpuscular  theory  of  light.  The  sun  and 
all  other  luminous  bodies  radiated  light,  they  thought,  be- 
cause they  ejected  minute  corpuscles  of  light  matter  in 
all  directions.  If,  now,  these  small  corpuscles  hit  against 
the  dust  particles  in  the  comets'  tails,  the  dust  particles 
would  be  carried  away  with  them,  and  their  repulsion  by 

the  sun  would  become  intelligible.     It  is  characteristic 

95 


WORLDS    IN   THE   MAKING 

that  Newton  would  not  admit  this  explanation  of  Kepler's, 
although  he  shared  Kepler's  opinion  on  the  nature  of  light. 
According  to  Newton,  the  deviation  of  the  tails  of  com- 
ets from  his  law  of  general  attraction  was  only  apparent. 
The  tails  of  comets,  he  argued,  behaved  like  the  columns 
of  smoke  rising  from  a  chimney,  which,  although  the  gases 
of  combustion  are  attracted  by  the  earth,  yet  ascend 
because  they  are  lighter  than  the  surrounding  air.  This 
view,  which  has  been  characterized  by  Newcomb  as  no 
longer  to  be  seriously  taken  into  consideration,  demon- 
strates the  strong  tendency  of  Newton  to  explain  every- 
thing with  the  aid  of  his  law. 

The  astronomers  followed  faithfully  in  the  footsteps 
of  their  inimitable  master,  Newton,  and  they  brushed 
aside  every  phenomenon  which  would  not  fit  into  his 
system.  An  exception  was  made  by  the  famous  Euler, 
who,  in  1746,  expressed  the  opinion  that  the  waves  of 
light  exerted  a  pressure  upon  the  body  upon  which  they 
fell.  This  opinion,  however,  could  not  prevail  against 
the  criticisms  with  which  others,  and  especially  De  Mairan, 
assailed  it.  That  Euler  was  right,  however,  was  proved 
by  Maxwell's  great  theoretical  treatise  on  the  nature  of 
electricity  (1873).  He  showed  that  rays  of  heat — and 
the  same  applies,  as  Bartoli  established  in  1876,  to 
radiations  of  any  kind — must  exercise  a  pressure  just  as 
great  as  the  amount  of  energy  contained  in  a  unit  vol- 
ume, by  virtue  of  their  radiation.  Maxwell  calculated 
the  magnitude  of  this  pressure,  and  he  found  it  so  small 
that  it  could  hardly  have  been  demonstrated  with  the 
experimental  means  then  at  our  disposal.  But  this  dem- 
onstration has  since  been  furnished,  with  the  aid  of 
measurements  obtained  in  a  vacuum,  by  the  Russian 
Lebedeff  and  by  the  Americans  Nichols  and  Hull  (1900, 
1901).  They  have  found  that  this  pressure,  the  so-called 

96 


THE   RADIATION   PRESSURE 

radiation  pressure,  is  exactly  as  great  as  Maxwell  pre- 
dicted. 

In  spite  of  Maxwell's  great  authority,  astronomers  quite 
overlooked  this  important  law  of  his.  Lebedeff,  indeed, 
tried  in  1892  to  apply  it  to  the  tails  of  comets,  which  he 
regarded  as  gaseous;  but  the  law  is  not  applicable  in  this 
case.  As  late  as  the  year  1900,  shortly  before  Lebedeff 
was  able  to  publish  his  experimental  verification  of  this 
law,  I  attempted  to  prove  its  vast  importance  for  the  ex- 
planation of  several  celestial  phenomena.  The  magni- 
tude of  the  radiation  pressure  of  the  solar  atmosphere 
must  be  equivalent  to  2.75  milligrammes  if  the  rays  strike 
vertically  against  a  black  body  one  square  centimetre  in 
area.  I  also  calculated  the  size  of  a  spherule  of  the  same 
specific  gravity  as  water,  such  that  the  radiation  pressure 
to  which  it  would  be  exposed  in  the  vicinity  of  the  sun 
would  balance  the  attraction  by  the  sun.  It  resulted 
that  equilibrium  would  be  established  if  the  diameter  of 
the  sphere  were  0.0015  mm.  A  correction  supplied  by 
Schwarzschild  showed  that  the  calculation  was  only  valid 
wlien  the  sphere  completely  reflects  all  the  rays  which  fall 
upon  it.  If  the  diameter  of  the  spherule  be  still  smaller, 
the  radiation  pressure  will  prevail  over  the  attraction,  and 
such  a  sphere  would  be  repelled  by  the  sun.  Owing  to 
the  refraction  of  light,  this  will,  according  to  Schwarz- 
schild, further  necessitate  that  the  circumference  of  the 
spherule  should  be  greater  than  0.3  time  the  wave-length 
of  the  incident  rays.  When  the  sphere  becomes  still  small- 
er, gravitation  will  once  more  predominate.  But  spherules 
whose  sizes  are  intermediate  between  these  two  limits  will 
be  repelled.  It  results,  therefore,  that  molecules,  which 
have  far  smaller  dimensions  than  those  mentioned,  will 
not  be  repelled  by  the  radiation  pressure,  and  that 
therefore  Maxwell's  law  does  not  hold  for  gases.  When 

97 


WORLDS   IN   THE   MAKING 

the  circumference  of  the  spherule  becomes  exactly  equal 
to  the  wave-length  of  the  radiation,  the  radiation  press- 
ure will  act  at  its  maximum,  and  it  will  then  surpass 
gravity  not  less  than  nineteen  times.  These  calculations 
apply  to  all  spheres,  totally  reflecting  the  light,  of  a  spe- 
cific gravity  like  water,  and  to  a  radiation  and  attraction 
corresponding  to  that  of  the  sun.  Since  the  sunlight  is 
not  homogeneous,  the  maximum  effect  will  somewhat  be 
diminished,  and  it  is  nearly  equal  to  ten  times  the  grav- 
ity for  spheres  of  a  diameter  of  about  0.00016  mm.1 

Before  we  had  recourse  to  the  radiation  pressure  for 
the  explanation  of  the  repulsion  phenomena  such  as  have 
been  observed  in  the  tails  of  comets,  it  was  generally  be- 
lieved with  Zollner  that  the  repulsion  was  due  to  electrical 
forces.  Electricity  undoubtedly  plays  an  important  part 
in  these  phenomena,  as  we  shall  see.  The  way  in  which 
it  acts  in  these  instances  was  explained  by  a  discovery  of 
C.  T.  R.  Wilson  in  1899.  Gases  can  in  various  ways  be 
transformed  into  conductors  of  electricity  which  as  a  rule 
they  do  not  conduct.  The  conducting  gases  are  said  to 
be  ionized — that  is  to  say,  they  contain  free  ions,  minute 
particles  charged  with  positive  or  negative  electricity. 
Gases  can  be  ionized,  among  other  ways,  by  being  ra- 
diated upon  with  Rontgen  rays,  kathode  rays,  or  ultra- 
violet light,  as  well  as  by  strong  heat.  Since  the  light 
of  the  sun  contains  a  great  many  ultra-violet  radia- 
tions, it  is  indisputable  that  the  masses  of  gases  in 
the  neighborhood  of  the  sun  (e.g.,  probably  in  comets 
when  they  come  near  the  sun)  will  partly  be  ionized, 

1  One  centimetre  of  water  contains  470  billions  of  these  spheres. 
Such  a  little  drop  of  water,  again,  contains  96  millions  of  molecules, 
and  there  are  probably  organisms  which  are  smaller  than  these 
drops.  Compare  the  experiments  with  ultra-microscopic  organisms 
by  E.  Raehlmann,  N.  Gaidukow,  and  others. 

98 


THE  RADIATION   PRESSURE 

and  will  contain  both  positive  and  negative  ions.  Ion- 
ized gases  are  endowed  with  the  remarkable  capability 
of  condensing  vapors  upon  themselves.  Wilson  showed 
that  this  property  is  possessed  to  a  higher  degree  by  the 
negative  ions  than  by  the  positive  ions  (in  the  condensa- 
tion of  water  vapor).  If  there  are,  therefore,  water 
vapors  in  the  neighborhood  of  the  sun  which  can  be  con- 
densed by  cooling,  drops  of  water  will,  in  the  first  instance, 
be  condensed  upon  the  negative  ions.  When  these  drops 
are  afterwards  repelled  by  the  radiation  pressure,  or  when 
they  sink,  owing  to  gravity,  as  drops  of  rain  sink  in  the 
terrestrial  atmosphere,  they  will  carry  with  them  the 
charge  of  the  negative  ions,  while  the  corresponding  posi- 
tive charge  will  remain  behind  in  the  gas  or  in  the  air. 
In  this  way  the  negative  and  positive  charges  will  become 
separated  from  each  other,  and  electric  discharges  may 
ensue  if  sufficiently  large  quantities  of  opposite  electricity 
have  been  accumulated.  By  reason  of  these  discharges 
the  gases  will  become  luminescent,  although  their  tem- 
perature may  be  very  low.  Stark  has  even  shown  that 
low  temperatures  are  favorable  for  the  display  of  a  strong 
luminosity  in  electric  discharges. 

We  have  stated  that  Kepler,  as  early  as  the  beginning 
of  the  seventeenth  century,  came  to  the  conclusion  that 
the  tails  of  comets  were  repelled  by  the  sun.  Newton 
indicated  how  we  might,  from  the  shape  of  the  comets' 
tails,  calculate  their  velocity.  The  best  way,  however, 
is  to  determine  this  velocity  by  direct  observation.  The 
comets'  tails  are  not  so  uniform  in  appearance  as  they 
are  generally  represented  in  illustrations,  but  they  often 
contain  several  luminous  nuclei  (Fig.  33),  whose  motions 
can  be  directly  ascertained. 

From  a  study  of  the  movements  of  comets'  tails,  Olbers 
concluded,  about  the  beginning  of  the  last  century,  that  the 

99 


WORLDS    IN   THE   MAKING 

repulsion  of  the  comets'  tails  by  the  sun  is  inversely  pro- 
portional to  the  square  of  their  distance — tMat  is  to  say, 
that  the  force  of  the  repulsion  is  subject  to  the  same  law 
as  the  force  of  gravitation.  We  can,  therefore,  express 


Fig.  33. — Photograph  of  Roerdanrs  comet   (1893   II.),  suggesting 
several  strong  nuclei  in  the  tail 

the  repulsion  effect  in  unfts  of  solar  gravitation,  and  this 
has  generally  been  done.  That  the  radiation  pressure 
will  in  the  same  manner  change  with  the  distance  is  only 

natural.     For  the  radiation  against  the  same  surface  is 

100 


THE  RADIATION   PRESSURE 


Fig.  34.— Photograph  of  Swift's  comet  (1892  I.) 


also  inversely  proportional  to  the  square  of  the  distance 
from  the  radiating  body,  the  sun. 

In  the  latter  part  of  the  past  century  the  Russian  as- 
tronomer Bredichin  conducted  a  great  many  measure- 
ments on  the  magnitude  of  the  forces  with  which  comets' 
tails  are  repelled  by  the  sun.  He  considered  himself,  on 
the  strength  of  these  measurements,  justified  in  dividing 
comets'  tails  into  three  classes.  In  the  first  class  the 
repulsion  was  19  times  stronger  than  gravitation;  in  the 
second  class,  from  3.2  to  1.5  times  stronger;  and  in  the 
third  class,  from  1.3  to  1  times  stronger.  Still  higher 

values  have,  however,  been  deduced  -for  several  comets. 

101 


i.  AJV.ORLDS  IN  THE  MAKING 

Thus  Hussey  found  for  the  comet  of  1893  (Roerdam's 
comet,  1893  II. ,  Fig.  33)  a  repulsion  37  times  as  strong 
as  gravitation;  and  Swift's  comet  (1892  I.)  yields  the 
still  higher  value  of  40.5  (Fig.  34).  Some  comets  show 
several  tails  of  different  kinds,  as  the  famous  comet  of 
Donati  (Fig.  35).  Its  two  almost  straight  tails  would 
belong  to  the  first  class,  and  the  more  strongly  developed 
and  curved  third  tail  to  the  second  class. 

Schwarzschild,  as  already  stated,  calculated  that  small 
spherules   reflecting   all  the   incident  light  and  of  the 


Fig.  35.— Donati's  comet  at  its  greatest  brilliancy  in  1858 

specific  gravity  of  water  would  be  repelled  by  the  sun 
with  a  force  that  might  balance  ten  times  their  weight. 
For  a  spherule  absorbing  all  the  light  falling  upon  it 

102 


THE   RADIATION   PRESSURE 

this  figure  would  be  reduced  to  five  times  the  weight. 
The  small  particles  of  comets  which,  according  to  spec- 
troscopic  observations,  probably  consist  of  hydrocarbons 
are  not  perfectly  absorbing,  but  they  permit  certain  rays 
of  the  sun  to  pass.  A  closer  calculation  shows  that  in 
this  case  forces  of  about  3.3  times  the  gravity  would 
result. 

Larger  spherules  yield  smaller  values.  Bredichin's  sec- 
ond and  third  classes  would  thus  be  well  adapted  to  meet 
the  requirements  which  the  radiation  pressure  demands. 

It  is  more  difficult  to  explain  how  such  great  forces 
of  repulsion  as  those  of  the  first  group  of  Bredichin  or  of 
the  peculiar  comets  of  Swift  and  of  Roerdam  can  occur. 
When  a  particle  or  drop  of  some  hydrocarbon  is  exposed 
to  powerful  radiation,  it  may  finally  become  so  intensely 
heated  that  it  will  be  carbonized.  It  will  yield  a  spongy 
coal,  because  gases  (chiefly  hydrogen)  will  escape  during 
the  carbonization,  and  the  particles  of  coal  will  resemble 
the  little  grains  of  coal-dust  which  fall  from  the  smoke- 
stacks of  our  steamboats,  and  which  afterwards  float  on 
the  surface  of  the  water.  It  is  quite  conceivable  that  such 
spherules  of  coal  (consisting  probably  of  so-called  mar- 
guerites, felted  or  pearly  structures  resembling  chains  of 
bacilli)  may  have  a  specific  gravity  of  0.1,  if  we  make 
allowance  for  the  gases  they  include  (compare  page 
106.)  A  light-absorbing  drop  of  this  density  of  0.1  might, 
in  the  most  favorable  case,  experience  a  repulsion  forty 
times  as  strong  as  the  gravitation  of  the  sun.  In  this 
manner  we  can  picture  to  ourselves  the  possibility  of 
the  greatest  observed  forces  of  repulsion. 

The  spectra  of  comets  confirm  in  every  respect  the 
conclusions  to  which  the  theory  of  -the  radiation  pressure 
leads  up.  They  display  a  faint,  continuous  spectrum 

which  is  probably  due  to  sunlight  reflected  by  the  small 
8  103 


WORLDS    IN    THE    MAKING 


particles.  Besides  this,  we  observe,  as  already  men- 
tioned, a  spectrum  of  gaseous  hydrocarbons  and  cyano- 
gen. These  band  spec- 
tra are  due  to  electric 
discharges ;  for  they  are 
observed  in  comets 
whose  distance  from  the 
sun  is  so  great  that 
they  cannot  appear  lu- 
minous owing  to  their 
own  high  temperatures. 
In  the  tail  of  Swift's 
comet  banded  spectra 
have  been  observed  in 
portions  which  were 
about  five  million  kil- 
ometres from  the  nu- 
cleus. The  electric  dis- 
charges must  chiefly  be 
emitted  from  the  outer 
parts  of  the  tails,  where, 
according  to  the  laws 
of  static  electricity,  the 
electric  forces  would  be 
strongest.  For  this  rea- 
son the  larger  tails  of 
comets  look  as  if  they 
Fig.  36. — Imitation  of  comets'  tails,  were  enveloped  in  cloaks 

Experiment   by  Nichols   and  Hull.  of   jj   ht    of    &   more    in_ 

The  light  oi  an  arc-lamp  is  concen-  , 

trated  by  a  lens  upon  the  stream  of  tense    luminosity. 

finely  powdered  particles  When  a  comet  Comes 

nearer  to  the  sun,  other 

less  volatile  bodies    also  begin  to  evaporate.     We  then 
find  the  lines  of  sodium  and,  when  the  comet  comes  very 

104 


THE   RADIATION   PRESSURE 

close  to  the  sun,  also  the  lines  of  iron  in  its  spectrum. 
These  lines  are  evidently  produced  by  substances  which 
have  been  evaporated  from  the  nucleus  of  the  comet. 
Like  the  meteorites  falling  upon  our  earth,  the  nucleus 
will  consist  essentially  of  silicates,  and  particularly  of 
the  silicate  of  sodium,  and,  further,  of  iron. 

We  can  easily  imagine  how  the  tails  of  comets  change 
in  appearance.  When  a  comet  draws  near  to  the  sun, 
we  observe  that  matter  is  ejected  from  that  part  of 
the  nucleus  which  is  turned  towards  the  sun.  The 
case  is  analogous  to  the  formation  of  clouds  in  the  ter- 
restrial atmosphere  on  a  hot  summer  day.  The  clouds 
are  provided  with  a  kind  of  hood  which  envelops  like  a 
thin,  semi-spherical  veil  that  side  of  the  nucleus  which 
turns  to  the  sun.  Sometimes  we  observe  two  or  more 
hoods  corresponding  to  the  different  layers  of  clouds  in 
the  terrestrial  atmosphere.  From  the  farther  side  of  the 
hood  matter  streams  away  from  the  sun.  The  tails  of 
comets  are  usually  more  highly  developed  when  they  ap- 
proach the  sun  than  when  they  recede  from  it.  That  may 
be,  as-  has  been  assumed  for  a  long  time,  because  a  large 
part  of  the  hydrocarbons  will  become  exhausted  while  the 
comet  passes  the  sun.  We  have  also  noticed  that  the 
so-called  periodical  comets,  which  return  to  the  sun  at 
regular  intervals,  showed  at  every  reappearance  a  fainter 
development  of  the  tail.  Comets,  further,  shine  at  their 
greatest  brilliancy  in  periods  of  strong  solar-spot  activity. 
We  may,  therefore,  assume  that  in  those  periods  the  sur- 
roundings of  the  sun  are  charged  to  a  relatively  high 
degree  with  the  fine  dust  which  can  serve  as  a  condensa- 
tion nucleus  for  the  matter  of  the  comets'  tails.  It  is 
also  probable  that  in  such  periods  the  ionizing  radiation 
of  the  sun  is  more  pronounced  than  usual,  owing  to  the 
simultaneous  predominance  of  faculse. 

105 


WORLDS    IN   THE   MAKING 

Nichols  and  Hull  have  attempted  to  imitate  tails  of 
comets.  They  heated  the  spores  of  the  fungus  Lycoperdon 
bovista,  which  are  almost  spherical  and  of  a  diameter 
of  about  0.002  mm.,  up  to  a  red  glow,  and  they  thus  pro- 
duced little  spongy  balls  of  carbon  of  an  average  density 
of  0.1.  These  they  mixed  with  emery-powder  and  intro- 
duced them  into  a  glass  vessel  resembling  an  hour-glass 
(Fig.  36)  from  which  the  air  had  previously  been  ex- 
hausted as  far  as  possible.  They  then  caused  the  pow- 
dered mass  to  fall  in  a  fine  stream  into  the  lower  part  of 
the  vessel  while  exposing  it  at  the  same  time  to  the  con- 
centrated light  of  an  arc-lamp.  The  emery  particles  fell 
perpendicularly  to  the  bottom,  while  the  little  balls  of 
carbon  were  driven  aside  by  the  radiation  pressure  of  the 
light. 

We  also  meet  with  the  effects  of  the  radiation  pressure 
in  the  immediate  neighborhood  of  the  sun.  The  rectilinear 
extension  of  the  corona  streamers  to  a  distance  which 
has  been  known  to  exceed  six  times  the  solar  diameter 
(about  eight  million  km.)  indicates  that  repelling  forces 
from  the  sun  are  acting  upon  the  fine  dust.  Astronomers 
have  also  compared  the  corona  of  the  sun  with  the  tails 
of  comets,  and  Donitsch  would  class  it  with  Bredichin's 
comets'  tails  of  the  second  class.  It  is  possible  to  calcu- 
late the  mass  of  the  corona  from  its  radiation  of  heat  and 
light.  The  heat  radiated  has  been  measured  by  Abbot. 
At  a  distance  of  30,000  km.  from  the  photosphere,  the 
corona  radiated  only  as  little  heat  as  a  body  at  —55°  Cent. 
The  reason  is  that  the  corona  in  these  parts  consists  of  an 
extremely  attenuated  mist  whose  actual  temperature  can 
be  estimated  by  Stefan's  law  at  4300°  Cent.  The  corona 
must,  therefore,  be  so  attenuated  that  it  would  only 
cover  a  190,000th  part  of  the  sky  behind  it.  We  arrive 
at  the  same  result  when  we  calculate  the  amount  of 

100 


THE   RADIATION   PRESSURE 

light  radiated  by  the  corona;  this  radiation  is  of  the 
order  of  that  of  the  full  moon,  being  sometimes  smaller, 
sometimes  greater,  up  to  twice  as  great.  The  consider- 
ations we  have  been  offering  apply  to  the  most  intense 
part  of  the  corona,  the  so-called  inner  corona.  Accord- 
ing to  Turner,  its  light  intensity  outward  diminishes  in 
the  inverse  ratio  of  the  sixth  power  of  its  distance  from 
the  centre  of  the  sun.  At  the  distance  of  a  solar  radius 
(690,000  km.)  the  light  intensity  would  therefore  be  only 
1.6  per  cent,  of  the  intensity  near  the  surface  of  the 
sun. 

Let  us  assume  that  the  matter  of  the  corona  consists 
of  particles  of  just  such  a  size  that  the  radiation  pressure 
would  balance  their  weight  (other  particles  would  be  ex- 
pelled from  the  inner  corona);  then  we  find  that  the 
weight  of  the  whole  corona  of  the  sun  would  not  exceed 
twelve  million  metric  tons.  That  is  not  more  than  the 
weight  of  four  hundred  of  our  large  ocean  steamships 
(e.g.,  the  Oceanic),  and  only  about  as  much  as  the  quan- 
tity of  coal  burned  on  the  earth  within  one  week. 

That  the  mass  of  the  corona  must  be  extremely  rarefied 
has  already  been  concluded,  from  the  fact  that  comets 
have  wandered  through  the  corona  without  being  visibly 
arrested  in  their  motion.  In  1843  a  comet  passed  the 
sun's  surface  at  a  distance  of  only  one-quarter  the  sun's 
radius  without  being  disturbed  in  its  progress.  Moulton 
calculated  that  the  great  comet  of  1881,  which  approached 
the  sun  within  one-half  its  radius,  did  not  encounter  a 
resistance  of  more  than  one-fifty-thousandth  of  its  mass, 
and  that  the  nucleus  of  the  comet  was  at  least  five  million 
times  denser  than  the  matter  of  the  corona.  Newcomb 
has  possibly  expressed  the  degree  of  attenuation  of  the 
corona  in  a  somewhat  exaggerated  way  when  he  said 
that  it  contains  perhaps  one  grain  of  dust  per  cubic  kilo- 

107 


WORLDS    IN    THE   MAKING 

metre  (a  cube  whose  side  has  a  length  of  three-fifths  of 
a  mile). 

However  small  the  quantity  of  matter  in  the  corona 
may  be,  and  however  unimportant  a  fraction  of  this  mass 
may  pass  into  the  coronal  rays,  it  is  yet  certain  that  there 
is  a  constant  loss  of  finely  divided  matter  from  the  sun. 
The  loss,  however,  is  not  greater  than  the  supply  of  mat- 
ter (compare  below) — -namely,  about  300  thousand  mill- 
ions of  tons  in  a  year — so  that  during  one  billion  years 
not  even  one-six-thousandth  of  the  solar  mass  (2X1027 
tons)  will  be  scattered  into  space.  This  number  is  very 
unreliable,  however.  We  know  that  many  meteorites  fall 
upon  the  earth,  partly  as  compact  stones,  partly  as  the 
finest  dust  of  shooting-stars  which  flash  up  in  the  terres- 
trial atmosphere  rapidly  to  be  extinguished.  These  masses 
may  be  estimated  at  about  20,000  tons  per  year.  Accord- 
ing to  this  estimate,  the  rain  of  meteorites  which  falls  upon 
the  sun  may  amount  to  300  thousand  millions  of  tons  in  a 
year.  All  the  suns  have  emitted  matter  into  space  for 
infinite  ages,  and  it  seems,  therefore,  a  natural  inference 
that  many  suns  would  no  longer  be  in  existence  if  there 
had  not  been  a  supply  of  matter  to  make  up  for  this 
loss.  The  cold  suns  undergo  relatively  small  losses,  but 
receive  just  as  large  inflows  of  matter  as  the  warm  suns. 
As,  now,  our  sun  belongs  to  the  colder  type  of  stars,  it 
is  probable  that  the  loss  of  matter  from  the  sun  has  for 
this  reason  been  overestimated  by  being  presumed  to  be 
as  great  as  the  accession.  The  presence  of  dark  celestial 
bodies  may  compensate  for  this  overestimation. 

Whence  do  the  meteorites  come  ?  If  they  were  not 
constantly  being  created,  their  number  should  have 
diminished  in  the  course  of  ages;  for  they  are  gradually 
being  caught  up  by  the  larger  celestial  bodies.  It  is  not 
at  all  improbable  that  they  arise  from  the  accrescence 

108 


THE   RADIATION   PRESSURE 

of  small  particles  which  the  radiation  pressure  has  been 
driving  out  of  the  sun.  The  chondri,  which  are  so  char- 
acteristic of  meteorites,  display  a  structure  as  if  they 
had  grown  together  out  of  a  multitude  of  extremely 
fine  grains  (Fig.  37).  Nordenskio'ld  says:  "Most  meteoric 
iron  consists  of  an  extremely  delicate  texture  of  various 


Fig:.  37. — Granular  chondrum  from  the  meteorite  of  Sexes. 
Enlargement  1  :  70.     After  S.  Tschermak 


alloys  of  metals.  This  mass  of  meteoric  iron  is  often  so 
porous  that  it  oxidizes  on  exposure  to  the  air  like  spongy 
iron.  The  Pallas  iron,  when  cut  through  with  a  saw,  shows 
this  property,  which  is  so  distressing  for  the  collector.  The 

109 


WORLDS    IN   THE   MAKING 

iron  of  Cranbourne,  of  Toluca,  and  others — in  fact,  almost 
all  the  meteorites  with  a  few  exceptions — display  the  same 
texture.  It  all  indicates  that  these  cosmical  masses  of 
iron  were  built  up  in  the  universe  by  particle  being  piled 
upon  particle,  of  iron,  nickel,  phosphorus,  etc.,  analogous 
to  the  manner  in  which  one  atom  of  a  metal  coalesces 
with  another  atom  when  the  metal  is  galvanically  de- 
posited from  a  solution.  Most  of  the  stony  meteorites  pre- 
sent a  similar  appearance.  Apart  from  the  crust  of  slag 
on  the  surface,  the  stone  is  often  so  porous  and  so  loose 
that  it  might  be  used  as  a  filtering  material,  and  it  may 
easily  be  crumbled  between  the  fingers."  When  the 
electrically  charged  grains  of  dust  coalesce,  their  small 
electrical  potential  (of  about  0.02  volt)  may  increase 
considerably.  Under  the  influence  of  ultra-violet  light 
these  masses  of  meteorites  are  discharged  when  they  ap- 
proach the  sun,  as  Lenard  has  shown.  Their  negative 
charge  then  escapes  in  the  shape  of  so-called  electrons. 
Since,  now,  the  sun  loses  through  the  rays  of  the 
corona  large  multitudes  of  particles,  and  these  particles 
probably  carry,  according  to  Wilson,  negative  electricity 
with  them,  the  positive  charge  must  remain  behind  in 
the  stratum  from  which  the  coronal  rays  were  emitted, 
and  also  on  the  sun  itself.  If  this  charge  were  sufficiently 
powerful,  it  would  prevent  the  negatively  charged  par- 
ticles in  the  corona  from  escaping  from  the  sun,  and  all 
the  phenomena  which  we  have  ascribed  to  the  radiation 
pressure  would  cease.  By  the  aid  of  the  tenets  of  the 
modern  theory  of  electrons,  I  have  calculated  the  maxi- 
mum charge  that  the  sun  could  bear,  if  it  is  not  to  stop 
these  phenomena.  The  charge  would  amount  to  two 
hundred  thousand  millions  of  coulombs — not  by  any 
means  too  large  a  quantity  of  electricity,  as  it  would  only 

be  sufficient  to  decompose  twenty-four  tons  of  water. 

no 


THE   RADIATION   PRESSURE 

By  means  of  this  positive  charge  the  sun.  exerts  a 
vast  attractive  power  upon  all  negatively  charged  par- 
ticles which  come  near  it.  We  have  already  remarked 
that  the  grains  of  sun-dust  which  have  united  to  form 
meteorites  lose  under  the  influence  of  ultra-violet  light 
their  charge  in  the  shape  of  negative  electrons,  ex- 
tremely minute  particles,  of  which  perhaps  one  thousand 
weigh  as  much  as  one  atom  of  hydrogen  (1  gramme  of 
hydrogen  contains  about  1024  atoms,  corresponding,  to 
1027  electrons).  These  electrons  wander  about  in  space. 
When  they  approach  a  positively  charged  celestial  body 
they  are  attracted  by  it  with  great  force.  If  the  electrons 
were  moving  with  a  velocity  of  300  km.  per  second,  as  in 
Lenard's  experiment,  and  if  the  sun  were  charged  to 
one-tenth  the  maximum  amount  just  calculated,  it  would 
be  able  to  draw  up  all  the  electrons  whose  rectilinear 
path  (so  far  as  not  curved  by  the  sun's  attraction)  would 
lie  at  a  distance  from  the  sun  125  times  as  great  as  the 
distance  between  the  sun  and  its  most  remote  planet, 
Neptune,  and  3800  times  as  great  as  the  distance  between 
the  sun  and  the  earth,  which,  after  all,  would  only  be  one- 
sixtieth  of  the  distance  from  our  nearest  fixed-star  neigh- 
bor. The  sun  drains,  so  to  speak,  its  surroundings  of 
negative  electricity,  and  this  draining  effect  carries  to 
the  sun,  as  could  easily  be  proved,  a  quantity  of  electric- 
ity which  is  directly  dependent  upon  the  positive  charge 
of  the  sun.  Thus,  so  far  as  electricity  is  concerned, 
ample  provision  has  been  made  for  maintaining  equilib- 
rium between  the  income  and  expenditure  of  the  sun. 

When  an  electrical  particle  enters  into  a  magnetic  field 
it  describes  a  spiral  about  the  so-called  magnetic  lines  of 
force;  when  at  a  greater  distance,  the  particles  appear 

to  move  in  the  direction  of  the  lines  themselves.    The 

ill 


WORLDS    IN    THE   MAKING 

rays  of  the  corona  emanating  from  the  solar  poles  show 
a  distinct  curvature  like  that  of  the  lines  of  force  about 
a  magnet,  and  for  this  reason  the  sun  has  been  regarded 
as  a  big  magnet  whose  magnetic  poles  nearly  coincide 
with  the  geographical  poles.  The  coronal  rays  nearer 
the  equator  likewise  show  this  curvature  (compare  Fig. 
30).  The  repelling  force  of  the  radiation  pressure  there 
is,  however,  at  right  angles  to  the  lines  of  force  and  much 
stronger  than  the  magnetic  force,  so  that  the  rays  of  the 
corona  are  compelled  to  form  two  big  streams  flowing  in 
the  equatorial  direction.  This  is  especially  noticeable  at 
times  of  sun-spot  minima.  During  the  times  of  sun-spot 
maxima  the  strength  of  the  radiation  pressure  of  the 
initial  velocity  of  the  grains  of  dust  seem  to  predom- 
inate so  markedly  that  the  magnetic  force  is  relatively 
small. 

The  astronomers  tell  us  that  the  sun  is  only  a  star  of 
small  light  intensity  compared  to  the  prominent  stars 
which  excite  our  admiration.  The  sun  further  belongs 
to  a  group  of  relatively  cold  stars.  We  may  easily  im- 
agine, therefore,  that  the  radiation  pressure  in  the  vicin- 
ity of  these  larger  stars  will  be  able  to  move  much  larger 
masses  of  matter  than  in  our  solar  system.  If  the  dif- 
ferent stars  had  at  any  time  consisted  of  different  chemi- 
cal elements,  this  difference  would  have  been  equalized 
in  the  course  of  ages.  The  meteorites  may  be  regarded 
as  samples  of  matter  collected  and  despatched  from  all 
possible  divisions  of  space.  Now,  what  bodies  do  we 
find  in  them? 

In.  the  comets  (compare  page  104)  iron,  sodium,  car- 
bon, hydrogen,  and  nitrogen  (as  cyanogen)  play  the  most 
important  part.  We  know,  especially  from  the  researches 
of  Schiaparelli  that  meteorites  often  represent  fragments 

of  comets,  and  must  therefore  be  related  to  them.    Thus 

112 


THE  RADIATION   PRESSURE 

Biela's  comet,  which  had  a  period  oi  6.6  years,  has  dis- 
appeared since  1852 — it  had  divided  into  two  parts  in 
1844  -- 1845.  The  comet  was  rediscovered  in  a  belt  of 
meteorites  of  the  same  period  which  approaches  the  orbit 
of  the  earth  each  year  on  November  27.  Similar  rela- 
tions have  been  observed  with  regard  to  several  other 
swarms  of  meteorites.  We  know  also  that  the  just-men- 
tioned elements  which  spectrum  analysis  has  proved  to 
exist  in  comets  are  the  main  constituents  of  the  meteor- 
ites, which,  in  addition,  contain  the  metals  calcium,  mag- 
nesium, aluminium,  nickel,  cobalt,  and  chromium,  as 
well  as  the  metalloids  oxygen,  silicon,  sulphur,  phos- 
phorus, chlorine,  arsenic,  argon,  and  helium.  Their 
composition  strongly  recalls  the  volcanic  products  of 
so-called  basic  nature — that  is  to  say,  those  which  con- 
tain relatively  large  proportions  of  metallic  oxides,  and 
which  have  been  thought  for  good  reasons  to  hail  from 
the  deeper  strata  of  the  interior  of  the  earth.  Lockyer 
heated  meteoric  stones  in  the  electric  arc  to  incandescence 
and  found  their  spectra  to  be  very  similar  to  the  solar 
spectrum. 

We  therefore  draw  the  conclusion  that  these  messen- 
gers from  other  solar  systems  which  bring  us  samples 
of  their  chemical  elements  are  closely  related  to  our  sun 
and  to  the  interior  of  our  earth.  That  other  stare  and 
comets  are  essentially  composed  of  the  same  elements  as 
our  sun  and  earth,  spectrum  analysis  had  already  in- 
timated to  us.  But  various  metalloids,  like  chlorine, 
bromine,  sulphur,  phosphorus,  and  arsenic,  which  are  of 
importance  for  the  composition  of  the  earth,  have  so 
far  not  been  traced  in  the  spectra  of  the  celestial  bodies, 
nor  in  that  of  the  sun.  W^e  find  them  in  meteorites, 
however,  and  there  is  not  the  slightest  doubt  that  we 
must  likewise  count  them  among  the  essential  constitu- 

113 


WORLDS    IN   THE   MAKING 

ents  of  the  sun  and  other  celestial  bodies.  It  is  with  diffi- 
culty, however,  that  the  metalloids  can  be  made  to  ex- 
hibit their  spectra,  and  this  is  manifestly  the  reason  why 
spectrum  analysis  has  not  yet  succeeded  in  establishing 
their  presence  in  the  heavens.  As  regards  the  recently 
discovered  so-called  noble  gases  helium,  argon,  neon,  kryp- 
ton, and  xenon,  their  presence  in  the  chromosphere  has 
been  discovered  on  spectrograms  taken  during  eclipses  of 
the  sun  (Stassano).  According  to  Mitchell,  however, 
these  statements  would  appear  to  be  somewhat  uncer- 
tain as  to  krypton  and  xenon. 

The  small  particles  of  dust  which  the  radiation  press- 
ure drives  out  into  space  to  all  possible  distances  from 
the  sun  and  the  stars  may  hit  against  one  another  and 
may  accumulate  to  larger  or  smaller  aggregates  in  the 
shape  of  cosmical  dust  or  meteorites.  These  aggregates 
will  partly  fall  upon  other  stars,  planets,  comets,  or 
moons,  and  partly — and  this  in  very  great  multitudes — • 
they  will  float  about  in  space.  There  they  may,  together 
with  the  larger  dark  celestial  bodies,  form  a  kind  of 
haze,  which  partly  hides  from  us  the  light  of  distant 
celestial  bodies.  Hence  we  do  not  see  the  whole  sky 
covered  with  luminous  stars,  which  would  be  the  case 
if,  as  we  may  surmise,  the  stars  were  uniformly  distrib- 
uted all  through  the  infinite  space  of  the  universe,  and 
if  there  were  no  obstacle  to  their  emission  of  light.  If 
there  were  no  other  celestial  bodies  of  very  low  tempera- 
ture and  very  large  dimensions  which  absorbed  the  heat 
of  the  bright  suns,  the  dark  celestial  bodies,  the  meteor- 
ites, and  the  dark  cosmical  dust  would  soon  be  so  strongly 
heated  by  solar  radiation  that  they  would  themselves 
turn  incandescent,  and  the  whole  dome  of  the  sky  would 
appear  to  us  like  one  glowing  vault  whose  hot  radiation 
down  to  the  earth  would  soon  burn  every  living  thing. 

114 


THE   RADIATION   PRESSURE 

These  other  cold  celestial  bodies  which  absorb  the 
solar  rays  without  themselves  becoming  hot  are  known  as 
nebulae.  More  recent  researches  make  us  believe  that 
these  peculiar  celestial  bodies  occur  nearly  everywhere  in 
the  sky.  The  wonderful  mechanism  which  enables  them 
to  absorb  heat  without  raising  their  own  temperature  will 
be  explained  later  (in  Chapter  VII.).  As  these  cold  nebu- 
lae occupy  vast  portions  of  space,  most  of  the  cosmical 
dust  must  finally,  in  its  wanderings  through  infinite  space, 
stray  into  them.  This  dust  will  there  meet  masses  of 
gases  which  stop  the  penetration  of  the  small  corpuscles. 
As  the  dust  is  electrically  charged  (particularly  with  nega- 
tive electricity),  these  charges  will  also  be  accumulated 
in  the  outer  layers  of  the  nebulae.  This  will  proceed  until 
the  electrical  tension  becomes  so  strong  that  discharges 
are  started  by  the  ejection  of  electrons.  The  surround- 
ing gases  will  therefore  be  rendered  luminescent,  although 
their  temperature  may  not  much  (perhaps  by  50°)  exceed 
absolute  zero,  —273°  Cent.,  and  in  this  way  we  are  en- 
abled to  observe  these  nebulae.  Most  of  the  particles 
will  be  stopped  before  they  have  had  time  to  penetrate 
very  deeply  into  a  nebula,  and  it  will  therefore  principally 
be  the  outer  portions  of  the  nebulae  which  send  their 
light  to  us.  That  would  conform  to  Herschel's  descrip- 
tion of  planetary  nebulae,  which  display  no  greater  lu- 
minosity in  their  centres,  but  which  shine  as  if  they 
formed  hollow  spherical  shells  of  nebulous  matter.  It 
is  very  easy  to  demonstrate  that  only  substances,  such  as 
helium  and  hydrogen,  which  are  most  difficult  to  condense, 
can  at  this  low  temperature  exist  in  gaseous  form  to  any 
noticeable  degree.  The  nebulae,  therefore*  shine  almost 
exclusively  in  the  light  of  these  gases.  There  occurs  in 
the  nebulae,  in  addition  to  these  gases,  a  mysterious  sub- 
stance, nebulium,  whose  peculiar  spectrum  has  not  been 

115 


WORLDS    IN    THE   MAKING 

found  on  the  earth  nor  in  the  light  of  stars.  For- 
merly the  character  of  the  nebular  spectrum  was  ex- 
plained by  the  assumption  either  that  no  other  bodies 
occurred  in  nebulae  than  the  substances  mentioned,  or 
that  all  the  other  elements  in  them  were  decomposed 
into  hydrogen — helium  was  not  known  then.  The  sim- 
ple explanation  is  that  only  the  gases  of  the  outer  layer 
of  the  nebulae  are  luminous.  How  their  interiors  are  con- 
stituted, we  do  not  know. 

It  has  been  objected  to  the  view  just  expressed  that 
the  whole  sky  should  glow  in  a  nebulous  light,  and  that 
even  the  outer  atmosphere  of  the  earth  should  display 
such  a  glow.  But  hydrogen  and  helium  occur  only  very 
sparely  in  the  terrestrial  atmosphere.  We  find,  how- 
ever, another  light,  the  so-called  auroral  line,  which  may 
possibly  be  due  to  krypton  in  our  atmosphere.  Which- 
ever way  we  turn  the  spectroscope  on  a  very  clear  night, 
especially  in  the  tropics,  we  observe  this  peculiar  green 
line.  It  was  formerly  considered  to  be  characteristic  of 
the  Zodiacal  Light,  but  on  a  closer  examination  it  has 
been  traced  all  over  the  sky,  even  where  the  Zodiacal 
Light  could  not  be  observed.  One  of  the  objections  to 
our  view  is  therefore  unjustified. 

As  regards  the  other  objection,  we  have  to  remark 
that  any  light  emission  must  exceed  a  certain  minimum 
intensity  to  become  visible.  There  may  be  nebulae,  and 
they  probably  constitute  the  majority,  which  we  cannot 
observe  because  the  number  of  electrically  charged  par- 
ticles rushing  into  them  is  far  too  insignificant.  A  con- 
firmation of  this  view  was  furnished  by  the  flashing-up 
of  the  new  star  in  Perseus  on  February  21  and  22,  1901. 
This  star  ejected  two  different  kinds  of  particles,  of  which 
the  one  kind  travelled  with  nearly  double  the  velocity  of 
the  other.  The  accumulations  of  dust  formed  two  spher- 

116 


THE   RADIATION    PRESSURE 

ical  shells  around  the  new  star,  corresponding  in  every 
respect  to  the  two  kinds  of  comets'  tails  of  Bredichin's 
first  and  second  classes,  which  we  have  sometimes  ob- 
served together  in  the  same  comet  (Fig.  35).  When  these 
dust  particles,  on  their  road,  hit  against  nebular  masses, 
the  latter  became  luminescent,  and  we  thereby  obtained 
knowledge  of  the  presence  of  large  stellar  nebulae  of 
whose  existence  we  previously  had  not  the  faintest 
suspicion.  Conditions,  no  doubt,  are  similar  in  other 
parts  of  the  heavens  where  we  have  not  so  far  discovered 
any  nebulae — we  believe,  because  of  the  small  number  of 
these  charged  particles  straying  about  in  those  parts. 
On  the  same  grounds  we  may  explain  the  variability  of 
certain  nebulae  which  formerly  appeared  quite  enigmatical. 


V 

THE  SOLAR  DUST  IN  THE  ATMOSPHERE— POLAR  LIGHTS 
AND  THE  VARIATIONS  OF  TERRESTRIAL  MAGNETISM 

WE  have  so  far  dwelt  on  the  effects  which  the  particles 
expelled  from  the  sun  and  the  stars  exert  on  distant 
celestial  bodies.  It  may  be  asked  whether  this  dust 
does  not  act  upon  our  own  earth.  We  have  already 
recognized  the  peculiar  luminescence  which  on  clear 
nights  is  diffused  over  the  sky  as  a  consequence  of  elec- 
trical discharges  of  this  straying  dust.  This  leads  to  the 
question  whether  the  magnificent  polar  lights,  which 
according  to  modern  views  are  also  caused  by  electric 
discharges  in  the  higher  strata  of  the  atmosphere,  are  not 
produced  by  dust  which  the  sun  sends  to  us.  It  will, 
indeed,  be  seen  that  we  can  in  this  way  explain  quite  a 
number  of  the  peculiarities  of  these  mysterious  phenomena 
which  have  always  excited  man's  imagination. 

We  know  that  meteorites  and  shooting-stars  are  ren- 
dered incandescent  by  the  resistance  which  they  encounter 
in  the  air  at  an  average  height  of  120  km.  (75  miles),  some- 
times of  150  and  200  km.  In  isolated  cases  meteorites 
are  supposed  to  have  become  visible  even  at  still  greater 
altitudes.  It  would  result  that  there  must  be  appre- 
ciable quantities  of  air  still  at  relatively  high  elevation, 
and  that  the  atmosphere  cannot  be  imperceptible  at  an 
altitude  of  less  than  100  km.,  as  was  formerly  assumed. 
Bodies  smaller  than  the  meteorites  as  well  as  the  solar  dust 

118 


THE  SOLAR  DUST  IN   THE  ATMOSPHERE 

we  have  spoken  of — which,  owing  to  their  minuteness  and 
to  the  strong  cooling  by  heat  radiation  and  conduction 
that  they  undergo  in  passing  through  the  atmosphere, 
could  never  attain  incandescence — would  be  stopped  at 
greater  heights.  We  will  assume  that  they  are  arrested 
at  a  mean  height  of  about  400  km.  (250  miles). 

The  masses  of  dust  which  are  expelled  by  the  sun  are 
partly  uncharged,  partly  charged  with  positive  or  nega- 
tive electricity.  Only  the  latter  can  be  connected  with 
the  polar  lights;  the  former  would  remain  dark  and  slow- 
ly sink  through  our  atmosphere  to  the  surface  of  the  earth. 
They  form  the  so-called  cosmical  dust,  of  whose  great 
importance  Nordenskiold  was  so  firmly  convinced.  He 
estimated  that  the  yearly  increase  in  the  weight -of  the 
earth  by  the  addition  of  the  meteorites  was  at  least  ten 
million  tons,  or  five  hundred  times  more  than  we  stated 
above  (page  108).  Like  Lockyer  and,  in  more  recent 
days,  Chamberlin,  he  believed  that  the  planets  were 
largely  built  up  of  meteorites. 

The  dust  reaching  the  earth  from  the  sun  would  not, 
were  it  not  electrically  charged,  amount  to  more  than 
200  tons  in  a  year.  Although  this  figure  may  be  far  too 
low,  yet  the  supply  of  matter  by  these  means  is  certainly 
very  small  in  comparison  with  the  20,000  tons  which  the 
earth  receives  in  the  shape  of  meteorites  and  shooting- 
stars.  But  owing  to  its  extremely  minute  distribution, 
the  effect  of  this  dust  is  very  important,  and  it  may  con- 
stitute a  much  greater  portion  of  the  finely  distributed 
cosmical  dust  in  the  highest  strata  of  the  atmosphere  than 
the  dust  introduced  by  falling  meteorites  and  shooting- 
stars. 

That  these  particles  exert  a  noticeable  influence  upon 
terrestrial  conditions,  in  spite  of  their  relatively  insig- 
nificant mass,  is  due  to  two  causes.  They  are  extremely 

9  119 


WORLDS    IN   THE   MAKING 

minute  and  therefore  remain  suspended  in  our  atmos- 
phere for  long  periods  (for  more  than  a  year  in  the  case 
of  the  Krakatoa  dust),  and  they  are  electrically  charged. 

In  order  to  understand  their  action  upon  the  earth, 
we  will  examine  how  the  terrestrial  conditions  depend 
upon  the  position  of  the  earth  with  regard  to  the  various 
active  portions  of  the  sun,  and  upon  the  change  of  the 
sun  itself  in  regard  to  its  emission  of  dust  particles.  For 
this  examination  we  have  to  avail  ourselves  of  extensive 
statistical  data;  for  only  a  long  series  of  observations  can 
give  us  a  clear  conception  of  the  action  of  solar  dust. 

These  particles  withdraw  from  the  sun  gases  which 
they  were  able  to  condense  on  their  surface,  and  which 
had  originally  been  in  the  chromosphere  and  in  the 
corona  of  the  sun.  The  most  important  among  these 
gases  is  hydrogen ;  next  to  it  come  helium  and  the  other 
noble  gases  which  Ramsay  has  discovered  in  the  atmos- 
phere, in  which  they  occur  in  very  small  quantities.  As 
regards  hydrogen,  Liveing  and  (after  him)  Mitchell  have 
maintained  that  it  is  not  produced  in  the  terrestrial  at- 
mosphere. Occasionally  it  is  certainly  found  in  volcanic 
gases.  Thus  hydrogen  escapes,  for  instance,  from  the 
crater  of  Kilauea,  on  Hawaii,  but  it  is  burned  at  once  in 
the  atmosphere.  If  hydrogen  were  present  in  the  at- 
mosphere, it  would  gradually  combine  with  the  oxygen 
to  water  vapor;  and  we  have  to  assume,  therefore,  that 
the  hydrogen  must  be  introduced  into  our  atmosphere 
from  another  source — namely,  from  the  sun.  Mitchell 
finds  in  this  view  a  strong  support  for  the  opinion  that 
solar  dust  is  always  trickling  down  through  our  atmos- 
phere. 

The  quantity  of  solar  dust  which  reaches  our  atmos- 
phere will  naturally  vary  in  proportion  with  the  erup- 
tive activity  of  the  sun.  The  quantity  of  dust  in  the 

120 


THE  SOLAR  DUST   IN   THE   ATMOSPHERE 

higher  strata  influences  the  color  of  the  light  of  the  sun. 
After  the  eruption  of  the  volcano  Rakata  on  Krakatoa, 
in  1883,  and  again,  though  to  a  lesser  degree,  after  the 
eruption  of  Mont  Pelee  on  Martinique,  red  sunsets  and 
sunrises  were  observed  all  over  the  globe.  At  the  same 
time,  another  phenomenon  was  noticed  which  could  be 
estimated  quantitatively.  The  light  of  the  sky  is  polar- 
ized with  the  exception  of  the  light  coming  from  a  few 
particular  spots.  Of  these  spots,  one  called  Arago's  Point 
is  situated  a  little  above  the  antipode  of  the  sun,  and 
another,  Babinet's  Point,  is  situated  above  the  sun.  If 
we  determine  the  elevation  of  these  points  above  the  hori- 
zon at  sunset,  we  find  in  accordance  with  the  theoretical 
deduction  that  this  elevation  is  greater  when  the  higher 
strata  of  the  atmosphere  are  charged  with  dust  (as  after 
the  eruption  of  Rakata)  than  under  normal  conditions. 
Busch,  a  German  scientist,  analyzed  the  mean  elevation 
of  these  points  (stated  in  degrees  of  arc)  at  sunset,  and 
found  the  following  peculiar  numbers: 


1886 

'87 

'88 

'89 

'90 

'91 

'92 

'93 

'94 

'95 

Mean 

Arago's  Point  .  .  . 

20.1 

19.7 

18.4 

17.8 

17.7 

20.6 

19.6 

20.2 

20.7 

18.8 

19.4 

Babinet's  Point  .  . 

23.9 

21.9 

17.9 

56.8 

15.4 

23.3 

21.5 

24.2 

23.3 

19.0 

20.7 

Sun-spot  Number 

21.1 

19.1 

6.7 

6.1 

6.5 

35.6 

73.8 

§4.9 

78.0 

63.9 

40.0 

There  is  a  distinct  parallelism  in  these  series  of  figures. 
Almost  simultaneously  with  the  sun-spot  maximum  the 
height  of  the  two  so-called  neutral  points  above  the 
horizon  attains  its  maximum  at  sunset,  and  the  same 
applies  to  the  minimum.  That  the  phenomena  in  the 
atmosphere  take  place  a  little  later  than  the  phenomena 
on  the  sun  which  caused  them  is  perhaps  only  natural. 
When  the  air  is  rich  in  dust,  or  when  it  is  strongly 
ionized  by  kathode  rays,  conditions  are  favorable  for 
the  formation  of  clouds.  This  can  be  observed,  for  in- 

121 


WORLDS   IN   THE   MAKING 

stance,  with  auroral  lights.  They  regularly  give  rise  to 
a  characteristic  cloud  formation,  so  much  so  that  Adam 
Paulsen  was  able  to  recognize  polar  lights  by  the  aid  of 
these  clouds  in  full  daylight.  Klein  has  compiled  a  table 
on  the  connection  between  the  frequency  of  the  higher 
clouds,  the  so-called  cirrus  clouds,  at  Cologne,  and  the 
number  of  sun-spots  during  the  period  1850-1900.  He 
demonstrates  that  during  this  half-century,  which  com- 
prises more  than  four  sun-spot  periods,  the  sun-spot 
maxima  fell  in  the  years  in  which  the  greatest  number  of 
cirrus  clouds  had  been  observed.  The  minima  of  the 
two  phenomena  are  likewise  in  agreement. 

A  similarly  intensified  formation  of  clouds  seems  also 
to  occur  on  Jupiter  when  sun-spots  are  frequent.  Vogel 
states  that  Jupiter  at  such  times  shines  with  a  whiter 
light,  while  at  sun-spot  minima  it  appears  of  a  deeper 
red.  The  deeper  we  are  able  to  peep  into  the  atmos- 
phere of  Jupiter,  the  more  reddish  it  appears.  During 
periods  of  strong  solar  activity  the  higher  portions  of 
Jupiter's  atmosphere  therefore  appear  to  be  crowded 
with  clouds. 

The  discharge  of  the  charged  solar  dust  in  our  atmos- 
phere calls  forth  the  polar  lights. 

The  polar  lights  occur,  as  the  name  indicates,  most  fre- 
quently in  the  districts  about  the  poles  of  the  earth.  They 
are,  however,  not  actually  more  frequent  the  nearer  we 
come  to  the  poles;  but  they  attain  a  maximum  of  fre- 
quency in  circles  which  enclose  the  magnetic  and  the 
geographical  poles.  The  northern  maximum  belt  passes, 
via  Cape  Tscheljuskin,  north  of  Novaja  Semi j a,  along  the 
northwestern  coast  of  Norway,  a  few  degrees  to  the  south 
of  Iceland  and  Greenland,  right  across  Hudson  Bay 'and 
over  the  northwestern  extension  of  Alaska.  When  we 
go  to  the  south  of  this  belt,  the  auroras,  or  boreal  lights, 

122 


POLAR   LIGHTS 

diminish  markedly.  They  are  four  times  less  frequent 
in  Edinburgh,  and  fifteen  times  less  frequent  in  London 
or  New  York,  than  in  the  Orkney  Islands  or  Lab- 
rador. 

Paulsen  divides  the  auroras  into  two  classes,  which 
behave  quite  differently  in  several  respects.  The  great 
difficulties  which  the  solution  of  the  problems  of  polar 
lights  has  so  far  offered  seem  to  a  large  extent  to  be 
due  to  the  fact  that  all  polar  lights  were  treated  as 
being  of  the  same  kind. 

The  polar  lights  of  the  first  class  do  not  display  any 
streamers.  They  cover  a  large  portion  of  the  sky  in  a 
horizontal  direction.  They  are  very  quiet,  and  their 
light  is  strikingly  constant.  As  a  rule,  they  drift  slowly 
towards  the  zenith,  and  they  do  not  give  rise  to  any 
magnetic  disturbances. 

These  polar  lights  generally  have  the  shape  of  an 
arch  whose  apex  is  situated  in  the  direction  of  the  mag- 
netic meridian  (Fig.  38).  Sometimes  several  arches  are 
grouped  one  above  another. 

Nordenskiold  observed  these  arches  quite  regularly 
during  the  polar  night  when  he  was  wintering  near  Pit- 
lekaj,  in  the  neighborhood  of  Bering  Sound.  Adam 
Paulsen  has  often  seen  them  on  Iceland  and  Greenland, 
which  are  situated  within  the  maximum  belt  spoken  of, 
where  northern  lights  are  very  common.  Occasionally, 
auroras  are  also  seen  farther  from  the  poles,  as  cir- 
cular arches  of  a  milky  white,  which  may  be  quite 
high  in  the  heavens. 

Sometimes  we  perceive  in  the  arctic  regions  that  large 
areas  of  the  heavens  are  covered  by  a  diffused  light  which 
might  best  be  compared  to  a  luminous,  transparent  cloud  ; 
the  darker  portions  in  it  probably  appear  dark  by  con- 
trast. This  phenomenon  was  frequently  observed  dur- 

123 


WORLDS    IN   THE   MAKING 


Fig.  38. — Arch-shaped  aurorae  borealis,  observed  by 
Nordenskiold  during  the  wintering  of  the  Vega  in 
Bering  Strait,  1879 


ing  the   Swedish  expedition  of   1882-1883,  near  Cape 
Thordsen. 

Masses  of  light  at  so  low  a  level  that  the  rocks  behind 
them  are  obscured  have  frequently  been  observed  to  float 
in  the  air,  especially  in  the  arctic  districts.  Thus  Lem- 
strom  saw  an  aurora  on  the  island  of  Spitzbergen  in  front 
of  a  wall  of  rock  only  300  m.  (1000  ft.)  in  height.  In 
northern  Finland  he  observed  the  auroral  line  in  the  light 
of  the  air  in  front  of  a  black  cloth  only  a  few  metres  dis- 
tant. Adam  Paulsen  counts  these  phenomena  also  as 
polar  lights  of  the  first  class,  and  he  regards  them  as 

124 


POLAR  LIGHTS 

phosphorescent  clouds  which  have  been  carried  down  by 
convection  currents  to  an  unusually  low  level  of  our  at- 
mosphere. 

Polar  lights  of  the  second  class  are  distinguished  by 
the  characteristic  auroral  rays  or  streamers.  Sometimes 
these  streamers  are  quite  separated  from  one  another 
(see  Fig.  39);  as  a  rule  they  melt  into  one  another,  es- 
pecially below,  so  as  to  form  draperies  which  are  so  easily 
moved  and  unsteady  that  they  appear  to  flutter  in  the 


Fig.  39. — Aurora  borealis,  with  radial  streamers 

wind  (Fig.  41.)  The  streamers  run  very  approximately 
in  the  direction  of  the  inclination  (magnetic  dip)  needle, 
and  when  -they  are  fully  developed  around  the  celestial 
dome  their  point  of  convergence  is  distinctly  discernible 
in  the  so-called  corona  (Fig.  40).  When  the  light  is  at 
its  greatest  intensity  the  aurora  is  traversed  by  numer- 
ous waves  of  light. 

The  draperies  are  very  thin.  Paulsen  watched  them 
sometimes  drifting  over  his  head  in  Greenland.  The  dra- 
peries then  appeared  foreshortened,  in  the  shape  of  striae  or 

125 


WORLDS    IN   THE   MAKING 

ribbons  of  light  in  convolutions.  These  polar  lights  in- 
fluence the  magnetic  needle.  When  they  pass  the  zenith 
their  influence  changes  sign,  so  that  the  deviation  of  the 
magnetic  needle  changes  from  east  to  west  when  the  rib- 


Fig.  40. — Aurora  with  corona,  observed   by 
Gyllenskiold  on  Spitzbergen,  1883 

bon  is  moving  from  north  to  south.  Paulsen  therefore 
concluded  that  negative  electricity  (kathode  rays)  was 
moving  downward  in  these  rays.  These  polar  lights  cor- 
respond to  violent  displacements  of  negative  electric- 
ity, while  polar  lights  of  the  first  class  appear  to  consist 
of  a  phosphorescent  matter  which  is  not  in  strong  agita- 
tion. The  streamers  may  penetrate  down  into  rather 
low  atmospheric  strata,  at  least  in  districts  which  are 
near  the  maximum  belt  of  the  northern  lights.  Thus 
Parry  observed  at  Port  Bowen  an  auroral  streamer  in 
front  of  a  cliff  only  214  m.  (700  ft.)  in  height. 

Polar  lights  of  the  first  order  may  pass  into  those 
of  the  second  order,  and  vice  versa.  We  frequently  see 
rays  suddenly  flash  out  from  the  arch  of  the  aurora,  most- 
ly downward,  but,  when  the  display  is  very  intense,  also 
upward.  On  the  other  hand,  the  violent  agitation  of  a 

126 


POLAR   LIGHTS 

"drapery  light"  may  cease,  and  may  give  way  to  a  dif- 
fused, steady  glow  in  the  sky.  The  polar  light  of  the  first 
class  is  chiefly  observed  in  the  arctic  regions.  To  it  cor- 
responds, in  districts  farther  removed  from  the  pole,  the 
diffused  light  which  appears  to  be  spread  uniformly  over 
the  heavens  and  which  gives  the  auroral  line.  • 

The  usually  observed  polar  lights  (speaking  not  only 
of  those  seen  on  arctic  expeditions)  belong  to  the  second 
class,  which  comprises  also  all  those  included  in  the  sub- 
joined statistics,  with  the  exception  of  the  auroral  dis- 
plays reported  from  Iceland  and  Greenland.  While  the 


Fig.  41. — Polar-light  draperies,  observed  in  Finnmarken,  northern 

Norway 

streamer  lights  distinctly  conform  to  the  11.1  years' 
period,  and  become  more  frequent  at  times  of  sun-spot 
maxima,  this  is  not  the  case,  according  to  Tromholt, 
with  the  auroras  of  Iceland  and  Greenland.  Their  fre- 

127 


WORLDS    IN   THE   MAKING 

quency,  on  the  contrary,  seems  to  be  rather  independent 
of  the  sun-spot  frequency.  Not  rarely  auroral  maxima 
corresponding  to  sun-spot  maxima  are  subdivided  into 
two  by  a  secondary  minimum.  This  phenomenon  is 
most  evident  in  the  polar  regions,  but  it  can  also  be 
traced  in  the  statistics  from  Scandinavia  and  from  other 
countries. 

Better  to  understand  the  nature  of  auroras,  we  will 
consider  the  sun's  corona  during  the  time  of  a  minimum 
year,  taking  as  an  example  the  year  1900  (compare  Fig. 
30).  The  rays  of  the  corona  in  the  neighborhood  of  the 
poles  of  the  sun  are  laterally  deflected  by  the  action  of 
the  magnetic  lines  of  force  of  the  sun.  The  small,  nega- 
tively charged  particles  have  evidently  only  a  low  veloc- 
ity, so  that  they  move  quite  close  to  the  lines  of  force  in 
the  neighborhood  of  the  solar  poles  and  are  concentrated 
near  the  equator.  There  the  lines  of  force  are  less  crowded 
—that  is  to  say,  the  magnetic  forces  are  weaker — and 
the  solar  dust  can  therefore  be  ejected  by  the  radiation 
pressure  and  will  accumulate  to  a  large  disk  expanding 
in  the  equatorial  plane.  To  us  this  disk  appears  like  two 
large  streams  of  rays  which  project  in  the  direction  of  the 
solar  equator.  Part  of  this  solar  dust  will  come  near  the 
earth  and  be  deflected  by  the  magnetic  lines  of  force 
of  the  earth;  it  will  hence  be  divided  into  two  streams 
which  are  directed  towards  the  two  terrestrial  magnetic 
poles.  These  poles  are  situated  below  the  earth's  crust, 
and  therefore  not  all  the  rays  will  be  concentrated  towards 
the  apparent  position  of  the  magnetic  poles  upon  the 
surface  of  the  earth.  It  is  to  be  expected  that  the  nega- 
tively charged  particles  coming  from  the  sun  will  chiefly 
drift  towards  that  district  which  is  situated  somewhat 
to  the  south  of  the  magnetic  north  pole,  when  it  is  noon 
at  this  pole.  When  it  is  midnight  at  the  magnetic 

128 


POLAR   LIGHTS 

pole,  most  of  the  negatively  charged  particles  will  be 
caught  by  the  lines  of  force  before  they  pass  the  geo- 
graphical north  pole,  and  the  maximum  belt  of  the  auroras 
will  for  this  reason  surround  the  magnetic  and  the  geo- 
graphical poles,  as  has  already  been  pointed  out  (com- 
pare page  122).  The  negatively  charged  solar  dust  will 
thus  be  concentrated  in  two  rings  above  the  maximum 
belts  of  the  polar  lights.  Where  the  dust  collides  with 
molecules  of  the  air,  it  will  produce  a  phosphorescent 
glow,  as  if  these  molecules  were  hit  by  the  electrically 
charged  particles  of  radium.  This  phosphorescent  glow 
rises  in  the  shape  of  a  luminous  arch  to  a  height  of 
about  400  km.  (250  miles) — according  to  Paulsen — and 
the  apex  of  this  arch  will  in  every  part  seem  to  lie  in 
the  direction  where  the  maximum  belt  is  nearest  to  the 
station  of  the  observer.  That  will  fairly  coincide  with 
the  direction  of  the  magnetic  needle. 

The  solar  corona  of  a  sun-spot  maximum  year  is  of  a 
very  different  appearance  (Fig.  31).  The  streamers  radi- 
ate straight  from  the  sun  in  almost  all  directions;  and  if 
there  be  some  privileged  directions,  it  will  be  those  above 
the  sun-spot  belts.  The  velocity  of  the  solar  dust  is  evi- 
dently so  great  that  the  streamers  are  no  longer  visibly 
deflected  by  the  magnetic  lines  of  force  of  the  sun.  Nor 
is  this  charged  dust  influenced  to  any  noticeable  degree 
by  the  magnetic  lines  of  force  of  the  earth.  It  will  in 
the  main  fall  straight  down  in  that  part  of  the  atmos- 
phere in  which  the  radiation  is  most  intense.  As  these 
"hard"  rays  of  the  sun1  seem  to  issue  from  the  faculse 
of  the  sun  which  are  most  frequent  in  maximum  sun-spot 

1  The  designations  "hard"  and  "soft"  streams  of  solar  dust  cor- 
respond to  the  terms  used  with  regard  to  kathode  rays.  The  soft 
rays  have  a  smaller  velocity,  and  are  therefore  more  strongly  deflected 
by  external  forces,  as,  for  instance,  magnetic  forces. 

129 


WORLDS   IN   THE   MAKING 

years,  some  polar  lights  will  also  be  seen  in  districts 
which  are  far  removed  from  the  maximum  belt  of  the 
auroras,  especially  when  the  number  of  sun-spots  is 
large.  The  opposite  relation  holds  for  the  " soft"  streams 
of  solar  dust  which  fall  near  the  maximum  belt  of  the 
polar  lights.  These  streams  occur  most  frequently  with 
low  sun-spot  frequency,  as  we  know  from  observations  of 
the  solar  corona.  Possibly  they  are  carried  along  by  the 
stream  of  harder  rays  in  maximum  years.  The  polar 
lights  corresponding  to  these  rays  therefore  attain  their 
maximum  with  few  sun-spots.  Hard  and  soft  dust 
streams  occur,  of  course,  simultaneously;  but  the  former 
predominate  in  maximum  sun-spot  years,  the  latter  in 
minimum  years. 

That  the  periodicity  of  the  polar  lights  in  regions  with- 
out, the  maximum  belt  follows  very  closely  the  periodicity 
of  the  sun-spots  was  shown  by  Fritz  as  early  as  1863. 
The  length  of  the  period  varies  between  7  and  16  years, 
the  average  being  11.1  years.  The  years  of  maxima  and 
minima  for  sun-spots  and  for  northern  auroras  are  the 
following : 

MAXIMUM    YEARS 

Sun-spots 1728  '39  '50  '62  '70  '78  '88  1804  '16  '30 

1837  '48  '60  '71  '83  '93  1905 

Northern  lights...  1730  '41  '49  '61  '73  '78  '88  1805  '19  '30 

1840  '50  '62  '71  '82  '93  1905 

MINIMUM   YEARS 

Sun-spots 1734  '45  '55  '67  '76  '85  '98  1811  '23  '34 

1844  '56  '67  '78  '89  1900 

Northern  lights...  1735  '44  '55  '66  '75  '83  '99  1811  '22  '34 

1844  '56  '66  '78  '89  1900 

There  are,  in  addition,  as  De  Mairan  proved  in  his 
classical  memoir  of  the  year  1746,  longer  periods  common 

130 


POLAR   LIGHTS 


to  both  the  number  of  sun-spots  and  the  number  of 
auroras.  According  to  Hansky,  the  length  of  this  period 
is  72  years;  according  to  Schuster,  33  years.  Very  pro- 
nounced maxima  occurred  at  the  beginning  and  the  end 
of  the  eighteenth  century,  the  last  in  the  year  1788;  after- 
wards auroras  became  very  rare  in  the  years  1800-1830, 
just  as  in  the  middle  of  the  eighteenth  century.  In 
1850,  and  particularly  in  1871,  there  were  strong  maxima; 
they  have  been  absent  since  then. 

The  estimates  of  the  heights  of  the  polar  lights  vary 
very  considerably.  The  height  seems  to  be  the  greater, 
on  the  whole,  the  nearer  the  point  of  observation  is  to  the 
equator,  which  would  well  agree  with  the  slight  deflec- 
tion of  the  kathode  rays  towards  the  surface  of  the 
earth  in  regions  which  are  farther  removed  from  the  pole. 
Gyllenskiold  found  on  Spitzbergen  a  mean  height  of  55 
km.;  Bravais,  in  northern  Norway,  100  to  200  km;  De 
Mairan,  in  central  Europe,  900  km.;  Galle,  again,  300 
km.  In  Greenland,  Paulsen  observed  northern  lights  at 
very  low  levels.  In  Iceland  he  fixed  the  apex  of  the  north- 
ern arch  which  may  be  considered  as  a  point  where  the 
charged  particles  from  the  sun  are  discharged  into  the 
air  at  about  400  km.  Not  much  reliance  can  be  placed 
upon  the  earlier  determinations;  but  the  heights  given 
conform  approximately  to  the  order  of  magnitude  which 
we  may  deduce  from  the  height  at  which  the  solar  dust 
will  be  stopped  by  the  terrestrial  atmosphere; 

The  polar  lights  possess,  further,  a  pronounced  yearly 
periodicity  which  is  easily  explicable  by  the  aid  of  the 
solar  dust  theory.  We  have  seen  that  sun-spots  are 
rarely  observed  near  the  solar  equator,  and  the  same  ap- 
plies to  solar  faculrc.  They  rapidly  increase  in  frequency 
with  higher  latitudes  of  the  sun,  and  their  maximum 
occurs  at  latitudes  of  about  fifteen  degrees.  The  equa- 

131 


WORLDS    IN    THE   MAKING 

torial  plane  of  the  sun  is  inclined  by  about  seven  degrees 
towards  the  plane  of  the  earth's  orbit.  The  earth  is  in 
the  equatorial  plane  of  the  sun  on  December  6th  and 
June  4th,  and  most  distant  from  it  three  months  later. 
We  may,  therefore,  expect  that  the  smallest  number  of 
solar-dust  particles  will  fall  on  the  earth  when  the  earth 
is  in  the  equator  of  the  sun — that  is,  in  December  and 
June — and  the  greatest  number  in  March  and  Septem- 
ber. These  relations  are  somewhat  disturbed  by  the 
twilight,  which  interferes  with  the  observation  of  auroras 
in  the  bright  summer  nights  of  the  arctic  region,  while 
the  dark  nights  of  the  winter  favor  the  observation  of 
these  phenomena.  The  distribution  of  the  polar  lights 
over  the  different  seasons  of  the  year  will  become  clear 
from  the  subjoined  table  compiled  by  Ekholm  and  my- 
self: 


January  .  . 

Sweden 
(1883-96) 
1056 

Norway 
(1861-95) 

251 

Iceland  and 
Greenland 
(1872-92) 

804 

United 
States 
(1871-93) 

1005 

Southern 
at  i  rone 
(1856-94) 

56 

February  . 

1173 

331 

734 

1455 

126 

March 

1312 

335 

613 

1396 

183 

April 

568 

90 

128 

1724 

148 

Mav     

170 

6 

1 

1270 

54 

June  

10 

0 

0 

10(>1 

40 

July  

54 

0 

0 

1233 

35 

August.  .  . 

191 

18 

40 

1210 

75 

September 
October 

1055 
1114 

209 
353 

455 
716 

1735 

1630 

120 
192 

November 

.  .  .     1077 

326 

811 

1240 

112 

December  . 

940 

260 

863 

912 

81 

Average 

number.        727 

181 

430 

1322 

102 

In  zones  where  the  difference  between  the  lengths  of 
day  and  night  of  the  different  seasons  is  not  very  great, 
as  in  the  United  States,  and  in  districts  in  which  the 
southern  light  is  observed  (about  latitude  40°  S.),  the 
chief  minimum  falls  in  winter:  on  the  northern  hemi- 
sphere, in  December ;  on  the  southern  hemisphere,  in  June 

132 


POLAR   LIGHTS 

or  July.  A  less  pronounced  minimum  occurring  in  the 
summer.  Twice  in  the  course  of  the  year  the  earth 
passes  through  the  plane  of  the  solar  equator.  During 
these  periods  a  minimum  of  solar  dust  trickles  down 
upon  the  earth,  and  that  period  is  characterized  by  a 
larger  number  of  polar  lights  which  is  distinguished  by  a 
higher  elevation  of  the  sun  above  the  horizon.  We  may 
expect  this;  for  most  solar  dust  will  fall  upon  that  por- 
tion of  the  earth  over  which  the  sun  is  highest  at  noon. 
The  two  maxima  of  March  or  April  and  of  September  or 
October,  when  the  earth  is  at  its  greatest  distance  from 
the  plane  of  the  solar  equator,  are  strongly  marked  in 
all  the  series,  except  in  those  for  the  polar  districts  Ice- 
land and  Greenland.  There  the  auroral  frequency  is 
solely  dependent  upon  the  intensity  of  the  twilight,  so 
that  we  find  a  single  maximum  in  December  and  the  cor- 
responding minimum  in  June.  More  recent  statistics 
(1891-1903)  indicate,  however,  a  minimum  in  December. 
For  the  same  reason  the  summer  minimum  in  countries 
of  high  latitudes,  like  Sweden  and  Norway,  is  very  much 
accentuated. 

Similar  reasons  render  it  difficult  for  most  localities 
to  indicate  the  daily  periodicity  of  the  polar  lights. 
Most  of  the  solar  dust  falls  about  noon,  and  most  polar 
lights  should  therefore  be  counted  a  few  hours  after 
noon,  just  as  the  highest  temperature  of  the  day  is 
reached  a  little  after  noon.  On  account  of  the  intense 
sunlight,  however,  this  maximum  can  only  be  estab- 
lished in  the  wintry  night  of  the  polar  regions,  and  even 
there  only  when  a  correction  has  been  made  for  the  dis- 
turbing effect  of  the  twilight.  In  this  way  Gyllenskiold 
found  a  northern-light  maximum  at  2.40  P.M.  for  Cape 
Thordsen,  on  Spitzbergen,  the  corresponding  minimum 

being  at  7.40  A.M.     In  other  localities  we  can  only  as- 

133 


WORLDS   IN   THE   MAKING 

certain  that  the  polar  lights  are  more  intense  and  more 
frequent  before  than  after  midnight.  In  central  Europe 
the  maximum  occurs  at  about  9  P.M.;  in  Sweden  and 
Norway  (in  latitude  60°  N.),  half  an  hour  or  an  hour 
later. 

A  few  other  periods,  approximately  of  the  length  of 
a  month,  have  .been  suggested  with  regard  to  polar 
lights.  A  period  lasting  25.93  days  predominates  in  the 
southern  lights,  where  the  maximum  exceeds  the  aver- 
age by  44  per  cent.  For  the  northern  lights  in  Norway 
the  corresponding  excess  percentage  is  23;  for  Sweden, 
only  II.1 

The  same  period  of  nearly  twenty-six  days  had  already 
been  pointed  out  for  a  long  series  of  other  especially  mag- 
netic phenomena  which,  as  we  shall  see,  are  very  closely 
connected  with  auroras,  and  it  had  also  been  found  in  the 
frequency  of  thunder-storms  and  in  the  variations  of  the 
barometer.  This  periodicity  has  often  been  thought  to  be 
connected  with  the  axial  rotation  of  the  sun.  The  Aus- 
trian scientist  Hornstein  has  even  gone  so  far  as  to  pro- 
pose that  the  length  of  this  period  should  be  carefully  de- 
termined, "  because  it  would  give  a  more  accurate  value 
for  the  rotation  of  the  sun  than  the  direct  determinations." 
We  know  now  that  the  length  of  the  solar  revolution  is  dif- 
ferent for  different  solar  altitudes,  a  circumstance  with 
which  observations  of  sun-spot  movements  at  different 
latitudes  had  already  made  Carrington  and  Sporer  famil- 
liar,  but  which  was  not  safely  established  before  Duner's 
spectroscopical  determination  of  the  movement  of  the  solar 

1  The  reason  is  that  in  the  southern  district  only  very  few,  and 
chiefly  the  most  intense,  auroras  are  recorded.  If  we  observe  very 
assiduously  in  a  large  country,  and  conduct  the  observations  at  differ- 
ent spots,  we  shall  find  polar  light  almost  every  night.  This  consid- 
eration partly  wipes  out  the  just-mentioned  differences. 

134 


VARIATIONS  OF  TERRESTRIAL   MAGNETISM 

photosphere.  Duner  found  the  following  sidereal  revo- 
lutions for  different  latitudes  of  the  sun  to  which  the  sub- 
joined sy nodical  revolution  would  correspond.  (By  side- 
real revolution  of  a  point  on  the  sun  we  understand  the 
time  which  elapses  between  the  two  moments  when  a 
certain  star  passes,  on  two  consecutive  occasions,  through 
the  meridian  plane  of  the  point — that  is  to  say,  through 
a  plane  laid  through  the  poles  of  the  sun  and  the  point 
in  question.  The  synodical  revolution  is  determined  by 
the  passage  of  the  earth  through  this  meridian.  On  ac- 
count of  the  proper  motion  of  the  earth  the  synodical 
period  is  longer  than  the  sidereal  period.) 

Latitude  on  the  sun  (degrees) ...       0        15        30        45        60        75 

Sidereal  revolution  (days) 25.4     26.4     27.6     30.0     33.9     38.5 

Synodical  revolution  (days) 27.3     28.5     29.9     32.7     37.4     43.0 

That  the  periods  of  rotation  of  the  solar  photosphere, 
and,  in  a  similar  way,  the  periods  of  the  spots,  the  faculse, 
and  the  prominences,  should  become  so  considerably 
longer  with  increasing  latitudes  is  one  of  the  most  mys- 
terious problems  of  the  physics  of  the  sun.  Something 
similar  applies  to  the  clouds  of  Jupiter,  but  the  differ- 
ence in  that  case  is  much  smaller — only  about  one  per 
cent.  The  clouds  of  our  atmosphere  behave  quite  dif- 
ferently, a  fact  which  is  explained  by  our  atmospheric 
circulation.1 

In  our  case,  of  course,  the  position  of  the  sun  with  re- 
gard to  the  earth — that  is  to  say,  the  synodical  period — can 

1  The  very  highest  strata  of  our  atmosphere  (at  levels  of  from 
20  to  80  km.,  15  to  50  miles)  may  perhaps  form  an  exception.  The 
luminous  clouds  which  were  observed  in  the  years  1883-1892  at 
Berlin  (after  the  eruption  of  Krakatoa),  and  which  were  floating  at 
a  very  high  level,  showed  a  drift  with  regard  to  the  surface  of  the 
earth  opposite  to  the  drift  of  the  cirrus  clouds,  which  are  directed 
eastward. 

'°  135 


WORLDS   IN   THE   MAKING 

alone  be  of  importance.  We  recognize  that  the  period 
of  25.93  days  does  not  at  all  agree  with  any  period  of 
the  solar  photosphere.  The  solar  equatorial  zone  differs 
least,  and  it  would  be  appropriate  to  reckon  with  this 
period,  since  the  earth  never  moves  very  far  from  the 
plane  of  the  solar  equator,  and  returns  to  that  plane,  at 
any  rate,  twice  in  the  course  of  a  year. 

But  there  is  another  peculiarity.  The  higher  a  point 
is  situated  in  the  atmosphere  of  the  sun,  the  shorter  is 
its  period.  Thus  the  synodical  period  of  the  faculse  near 
the  equator  is  on  an  average  26.06,  the  period  of  the  spots 
26.82,  of  the  photosphere  27.3  days.  Faculse  situated  at 
higher  levels  revolve  still  more  rapidly,  and  we  are  thus 
driven  to  the  conclusion  that  the  period  to  which  we 
have  alluded  agrees  with  the  period  of  the  faculse  situated 
at  higher  levels  in  the  equatorial  zone  of  the  sun,  and  is 
probably  dependent  upon  them.  That  would  conform 
to  our  ideas  concerning  the  physics  of  the  sun.  For  the 
faculse  are  produced  in  the  ascending  currents  of  gas  and 
at  rather  lower  levels  than  the  spherules  which  are  ex- 
pelled by  the  radiation  pressure.  This  radiation  pressure 
is  strongest  just  in  the  neighborhood  of  the  faculse. 

For  the  same  reason  the  repulsion  of  the  solar  dust 
becomes  particularly  powerful  when  the  faculse  are 
strongly  developed — that  is  to  say,  just  in  the  time  of 
pronounced  eruptive  activity  of  the  sun  which  is  char- 
acterized by  many  sun-spots. 

We  must  thus  imagine  that  the  radiation  of  the  sun 
will  be  stronger  in  times  of  strong  eruptive  activity  than 
during  the  days  of  low  sun-spot  frequencies.  Direct  ob- 
servations of  the  intensity  of  the  solar  radiation  which 
have  been  made  by  Saveljeff  in  Kieff  confirm  this  assump- 
tion. It  must  be  pointed  out,  however,  that  another 
phenomenon  investigated  by  Koppen  seems  to  contradict 

136 


VARIATIONS  OF  TERRESTRIAL   MAGNETISM 

this  conclusion.  Koppen  ascertained  that  in  our  tropics 
the  temperature  was  by  0.32°  Cent,  (nearly  0.6°  F.) 
lower  during  sun  -  spot  maxima  than  the  average,  and 
that  five  years  later,  a  year  before  the  sun-spot  mini- 
mum, it  reached  its  maximum  value  of  0.41°  Cent. 
(0.7°  F.)  above  the  average.  A  similar  peculiarity  can 
be  traced  to  other  zones,  but  on  account  of  the  less 
steady  climates  it  is  much  less  marked  there  than  in  the 
tropics.  A  French  physicist,  Nordmann,  has  fully  con- 
firmed the  observations  of  Koppen.  On  the  other  hand, 
Very,  an  American  astronomer,  has  found  that  the  tem- 
perature in  very  dry  (desert)  districts  of  the  tropics 
(near  Port  Darwin,  12°  28'  S.,  and  near  Alice  Springs, 
23°  38'  S.,  both  in  Australia)  is  higher  at  sun-spot  maxi- 
ma than  at  minima;  but  Very  was  in  this  research  guided 
merely  by  the  records  of  maximum  and  minimum  ther- 
mometers. From  Very's  investigation  it  would  appear 
that  the  solar  radiation  is  really  more  intense  with  larger 
sun-spot  numbers.1  This,  it  must  be  remarked,  is  only 
noticeable  in  exceedingly  dry  districts  in  which  there  is  no 
cloud  formation  worth  mentioning.  In  other  districts  the 
cloud  formation  which  accompanies  sun-spot  maxima  in- 
terferes with  the  simplicity  of  the  phenomena.  The  cool- 
ing effect  of  the  clouds  seems  in  these  cases  by  far  to  sur- 
pass the  direct  heating  effect  of  the  solar  rays,  and  in  this 
manner  Koppen's  conclusion  would  become  explicable.  If 
we  could  observe  the  temperatures  of  the  atmospheric 
strata  above  the  clouds,  their  variation  would  no  doubt 
be  in  the  same  degree  as  that  in  the  desert. 
Finally,  we  have  to  note  another  period  in  the  phe- 

1  According  to  Memery  (Bull.  Soc.  Astr.,  March  7,  1906,  p.  168)  an 
instantaneous  rise  of  temperature  is  observed  immediately  when  a 
sun-spot  is  first  seen,  and  the  temperature  sinks  again  when  the 
sun-spot  disappears. 

137 


WORLDS   IN   THE   MAKING 

nomena  of  the  polar  lights — the  so-called  tropical  month, 
whose  length  is  27.3  clays.  The  nature  of  this  period  is 
little  understood.  It  is  possibly  connected  with  the 


DECLINATION  '905 


4   f»t 


Fig.  42. — Curve  of  magnetic  declination  at  Kew,  near  Lon- 
don, on  November  15  and  1C,  1905.  The  violent  disturb- 
ance of  November  15,  9  P.M.,  corresponds  to  the  maximum 
intensity  of  the  aurora.  Compare  the  following  figure 


electric  charge  of  the  moon.  The  peculiarity  of  this 
period  is  that  it  acts  in  an  opposite  way  in  the  northern 
and  southern  hemispheres.  When  the  moon  is  above 
the  horizon,  it  seems  to  prevent  the  formation  of  polar 
lights;  but  for  this  case  the  difficulties  of  observation 
caused  by  the  moonlight  must,  of  course,  be  taken  into 
consideration. 

Celsius  and  Hiorter  observed  in  1741  that  the  polar 
lights  exercise  an  influence  on  the  magnetic  needle. 
From  this  circumstance  we  have  drawn  the  conclusion 
that  the  polar  lights  are  in  some  way  due  to  electric  dis- 
charges which  act  upon  the  magnetic  needle.  These 
magnetic  effects,  the  disturbances  of  the  otherwise  steady 
position  of  the  magnetic  needle,  are  not  influenced  by 
the  light  of  the  sun  and  moon,  and  can  therefore  be 

138 


VARIATIONS  OF  TERRESTRIAL   MAGNETISM 

studied  to  greater  advantage  than  auroras.  We  have 
already  pointed  out  that  it  is  only  the  aurora  of  the 
radial,  streamer  type  which  exerts  this  magnetic  influence 
(compare  Figs.  42  and  43). 

These  magnetic  variations  show  exactly  the  same 
periods  as  the  northern  lights  and  the  sun-spots.  As 
regards,  first,  the  long  period  of  11.1  years,  our  observa- 
tions prove  that  the  so-called  magnetic  disturbances  of 
the  position  of  the  magnetic  needle  faithfully  reflect  the 
variations  in  the  sun-spots.  This  connection  was  dis- 
covered in  1852  by  Sabine  in  England,  by  Wolf  in  Switzer- 
land, and  by  Gautier  in  France.  Even  the  more  irregu- 
lar diurnal  variations  in  the  magnetic  elements  are  sub- 
jected to  a  solar  period.  The  magnetic  needle  points 
in  our  districts  with  its  north  end  towards  the  north — not 
exactly,  though,  being  deflected  towards  the  west.  This 


HORIZONTAL  4NTENSITAT   /905 

/Vo>    ,S 


4                  a  ,0  A»r* 

>^'*> 


Fig.  43. — Curve  of  horizontal  intensity  at  Kew  on  November 
15  and  10,  1905.  On  November  15  a  magnificent  aurora 
was  observed  in  Galicia,  Germany,  France,  Norway,  Eng- 
land, Ireland,  and  Nova  Scotia,  with  a  maximum  at  9  P.M. 
The  polar  light  was  unusually  brilliant  as  early  as  6  P.M. 

western  deviation  or  declination  is  greatest  soon  after 
noon,  about  one  o'clock,  and  this  diurnal  change  is  greater 
in  summer  than  in  winter,  and  the  fluctuation  of  the 

139 


WORLDS    IN    THE   MAKING 

position  of  the  magnetic  needle  greater  in  daytime  than 
at  night-time.  It  is,  therefore,  manifest  that  we  have  to 
deal  with  some  solar  effect.  This  becomes  still  more  dis- 
tinct when  we  study  the  change  with  reference  to  the 
daily  variation  in  the  number  of  sun-spots.  In  the  sub- 
joined table  the  variation  in  the  declination  has  been 
compiled  for  Prague  for  the  years  1856  to  1889.  Only 
years  with  maxima  and  minima  of  sun-spots  and  of  mag- 
netic variations  have  been  noticed  in  this  table: 

'1&56     I860     1867      1871     1879     1884     1889 

Sun-spot  number.. 4. 3        95.7        7.3        139.1         3.4        63.7        6.3 

DAILY    VARIATIONS    IN    DECLINATION 
1856  1859  1867  1871  1878  1883  1889 

Observed  ...  5.98  10.36  6.95  11.43  5.65  8.34  5.99 
Calculated...  6.08  10.20  6.22  12.15  6.04  8.76  6.17 

We  see  that  the  maxima  and  minima  years  of  the  two 
phenomena  very  nearly  coincide.  The  accord  is  so  evi- 
dent that  we  may  calculate  the  diurnal  variation  as  pro- 
portional to  the  increase  in  the  number  of  sun-spots. 
This  is  shown  by  the  two  last  lines  of  the  table. 

The  yearly  variation  is  again  exactly  the  same  as  that 
of  polar  lights,  as  the  following  table  indicates,  in  which 
the  disturbances  of  magnetic  declination,  horizontal  in- 
tensity, and  vertical  intensity  are  compiled  for  Toronto, 
Canada;  for  comparison  the  means  of  these  three  magni- 
tudes are  added  for  Greenwich.  The  unit  of  this  table 
is  the- average  annual  variation: 

TORONTO 
Jan.        Feb.      Mar.        April        May      June      July      Aog.        Sept.         Oot.      Nor.      Dec. 

Declination.. 0.57  0.84  1.11  1.42  0.98  0.53  0.94  1.16  1.62  1.31  0.78  0.76 
Horizontal...  0.56  0.94  0.94  1.5O  0.90  0.36  0.61  0.75  1.71  J.48  098  0.58 
Vertical 0.57  0.74  1.08  1.49  1.12  0.50  0.71  1.08  1.61  1.29  0.75  0.61 

GREENWICH 

Jan.  Feb.         Mar.      April       May       June        July      Aug.      Sept.         Oct.          Nov.       Dec. 

Means.. 0.93     1.23     1.22     1.09     0.81     0.71     0.81     0.90     1.15     1.18      1.02     0.83 

140 


VARIATIONS   OF  TERRESTRIAL   MAGNETISM 

The  daily  variation  of  the  disturbances  has  been  an- 
alyzed by  Van  Bemmelen  for  the  period  1882-1893  and 
for  the  observatory  of  Batavia,  on  Java.  The  maxi- 
mum occurs  there  about  1  P.M.,  and  is  about  1.86  times 
as  great  as  the  average  value  for  the  day.  The  minimum 
of  0.48  occurs  at  11  P.M.  Between  8  P.M.  and  3  A.M. 
the  disturbances  are  almost  as  rare  as  about  11  o'clock 
at  night. 

The  variation  is  greatest  with  that  declination  which 
has  its  maximum  of  3.26  at  12  M.,  and  its  minimum  of 
0.14  at  11  P.M. 

The  period  of  almost  26  days  first  investigated  by  Horn- 
stein  has  also  been  refound  in  the  magnetic  variations 
and  disturbances  by  Broun,  Liznar,  and  C.  A.  Miiller. 
It  must  be  added,  however,  that  Schuster  does  not  con- 
sider these  data  as  in  any  way  conclusive. 

The  moon  has  also  a  slight  influence  upon  the  magnetic 
needle,  as  Kreil  proved  as  early  as  1841.  The  effect  is  in 
a  different  sign  in  the  northern  and  southern  hemi- 
spheres, and  may  be  likened  to  a  tidal  phenomenon. 

The  ultra-violet  rays  of  the  sun  are  strongly  absorbed 
by  the  atmosphere,  and  they  cause  an  ionization  of  the 
molecules  of  the  air.  This  ionization  is,  on  the  whole, 
more  marked  at  higher  altitudes.  The  ascending  air 
currents  carry  with  them  water  vapor  which  is  condensed 
on  the  ions,  particularly  on  the  negative  ions.  In  this 
way  most  clouds  become  negatively  charged;  this  inter- 
esting fact — i.  e.,  that  they  are  more  frequently  charged 
with  negative  than  with  positive  electricity — was  first 
proved  by  Franklin  in  his  kite  experiments.  When  the 
rain-drops  have  fallen,  the  air  above  remains  positively 
charged;  this  has  been  observed  during  balloon  ascen- 
sions. The  clouds  which  are  formed  at  high  levels  are 
most  strongly  charged;  for  this  reason  thunder-storms 

141 


WORLDS    IN    THE    MAKING 

over  land  occur  mostly  in  the  summer-time.  The  thun- 
der-storms also  show  the  26-day  period,  as  Bezold  has 
proved  for  southern  Germany,  and  Ekholm  and  myself 
have  shown  for  Sweden. 

A  vast  amount  of  material  concerning  these  questions 
and  magnetic  phenomena  in  particular  has  been  col- 
lected by  the  various  meteorological  observatories  and 
is  awaiting  analysis. 

Although  some  observers  like  Sidgreaves  question  the 
correlation  of  sun-spots  and  polar  lights  or  magnetic 
disturbances,  because  strong  spots  have  been  seen  on 
the  disk  of  the  sun  without  any  magnetic  disturbances 
having  been  noticed,  yet  the  view  predominates  that  the 
magnetic  disturbances  are  caused  by  sun-spots  when  the 
sun-spots  cross  the  central  meridian  of  the  sun  which  is 
opposite  the  earth.  Thus  Maunder  observed  a  magnetic 
storm  and  a  northern  light  succeeding  the  passage  of  a 
large  sun-spot  through  the  central  solar  meridian  on  the 
8th  to  the  10th  of  September,  1898.  The  magnetic  effect 
attained  its  maximum  about  twenty-one  hours  after  the 
passage  through  the  meridian. 

Similarly  Ricco  established  in  ten  instances,  in  which 
exact  determination  was  possible,  a  time  interval  of 
45.5  hours  on  an  average  between  the  meridian  passage 
of  a  spot  and  the  maximum  magnetic  effect.  Ricco 
also  submitted  to  an  analysis  the  data  which  Ellis  had 
collected  and  which  Maunder  had  investigated.  He 
found  for  these  instances,  on  an  average,  almost  exactly 
the  same  numbers,  the  time  interval  being  42.5  hours. 
That  would  correspond  to  a  mean  velocity  of  the  solar 
dust  of  from  910  to  980  km.  per  second.  On  the  other 
hand,  we  have  no  difficulty  at  all  in  calculating  the  time 
which  a  spherule  of  a  diameter  of  0.00016  mm.  (those 
particles  travel  fastest)  and  of  the  specific  gravity  of 

142 


VARIATIONS  OF  TERRESTRIAL   MAGNETISM 

water  would  need  in  order  to  reach  the  earth,  under  the 
influence  of  solar  gravitation  and  of  a  mechanical  radia- 
tion pressure  2.5  times  as  large  from  the  outside  of  the 
sun.  The  time  found,  56.1  hours,  corresponds  to  a  mean 
velocity  of  740  km.  per  second.  In  order  that  the  solar 
dust  may  move  with  the  velocity  calculated  by  Ricco,  its 
specific  gravity  should  be  less  than  1 — viz.,  0.66  and  0.57. 
This  value  looks  by  no  means  improbable,  when  we  as- 
sume that  the  spherules  consist  of  hydro-carbons  satu- 
rated with  hydrogen,  helium,  arid  other  noble  gases.  We 
should  also  arrive  at  larger  velocities  for  the  solar  dust, 
as  has  already  been  pointed  out  with  regard  to  the  tails 
of  comets,  when  we  presume  that  the  particles  consist 
of  felted  marguerites  of  carbon  or  silicates,  or  of  iron — 
materials  which  we  regard  as  the  main  constituents  of 
meteorites. 

It  should,  perhaps,  be  mentioned  that  the  most  in- 
tense spectrum  line  of  the  polar  lights  has  been  found 
to  be  characteristic  of  the  noble  gas  krypton.  As  this 
gas  is  found  only  in  very  small  quantities  in  our  atmos- 
phere, it  does  not  appear  improbable  that  it  has  been 
carried  to  us  together  with  the  solar  dust,  and  that  its 
spectrum  becomes  perceptible  during  the  discharge  phe- 
nomena. The  other  spectrum  lines  of  the  polar  lights 
belong  to  the  spectra  of  nitrogen,  argon,  and  of  the 
other  noble  gases.  The  volumes  of  noble  gases  which  are 
brought  into  our  terrestrial  atmosphere  in  this  manner 
would  in  any  case  be  exceedingly  small. 

The  electrical  phenomena  of  our  terrestrial  atmos- 
phere indirectly  possess  considerable  importance  for  or- 
ganic life  and,  consequently,  for  human  beings.  By  the 
electrical  discharges  part  of  the  nitrogen  is  made  to  com- 
bine with  the  oxygen  and  hydrogen  (liberated  by  the 
electric  decomposition  of  water  vapor)  of  our  air,  and  it 

143 


WORLDS   IN   THE   MAKING 

thus  forms  the  ammonia  compounds,  as  well  as  the  nitrates 
and  nitrites,  which  are  so  essential  to  vegetable  growth. 
The  ammonia  compounds  which  play  a  most  important 
part  in  the  temperate  zones  appear  especially  to  be  formed 
by  the  so-called  silent  discharges  which  we  connect  with 
auroras.  The  oxygen  compounds  of  nitrogen,  on  the 
other  hand,  seem  to  be  chiefly  the  products  of  the  violent 
thunder-storms  of  the  tropics.  The  rains  carry  these 
compounds  down  into  the  soil,  where  they  fertilize  the 
plants. 

The  supply  of  nitrogen  thus  fixed  amounts  in  the 
course  of  a  year  to  about  1.25  gramme  per  square  metre 
in  Europe,  and  to  almost  fourfold  that  figure  in  the 
tropics.  If  we  accept  three  grammes  as  the  average  num- 
ber for  the  whole  firm  land  of  the  earth,  that  would  mean 
3  tons  per  square  kilometre,  and  about  400  million  tons 
per  year  for  the  whole  firm  land  of  136  million  square 
kilometres.  A  very  small  portion  of  this  fixed  nitrogen, 
possibly  one-twentieth,  falls  on  cultivated  soil;  a  larger 
portion  will  help  to  stimulate  plant  growth  in  the  forests 
and  on  the  steppes.  We  may  mention,  for  comparison, 
that  the  nitrogen  contained  in  the  saltpetre  which  the 
mines  of  Chili  yield  to  us  has  risen  from  50,000  tons  in 
1880  to  120,000  tons  in  1890,  to  210,000  tons  in  1900, 
and  to  260,000  tons  in  1905.  The  nitrogen  produced  in 
the  shape  of  ammonium  salts  (sulphate)  by  the  gas-works 
of  Europe  amounts  to  about  one-quarter  of  the  last- 
mentioned  figure.  To  this  figure  we  have,  of  course,  to 
add  the  production  of  coal-gas  ammonia  in  the  United 
States  and  elsewhere.  Yet  even  allowing  for  this  item, 
we  shall  find  that  the  artificial  supply  of  combined  nitro- 
gen on  the  earth  does  not  represent  more  than  about  one- 
thousandth  of  the  natural  supply. 

As  the  nitrogen  contents  of  the  air  may  be  estimated 

144 


VARIATIONS  OF  TERRESTRIAL   MAGNETISM 

at  3980  billion  tons,  we  recognize  that  only  one  part  in 
three  millions  of  the  nitrogen  of  the  atmosphere  is  every 
year  fixed  by  electric  discharges,  presuming  that  the 
nitrogen  supply  to  the  sea  is  as  great  per  square  kilo- 
metre as  to  the  land.  The  nitrogen  thus  fixed  benefits 
the  plants  of  the  sea  and  of  the  land,  and  passes  back  into 
the  atmosphere  or  into  the  water  during  the  life  of  the 
plants  or  during  their  decay.  Water  absorbs  some  nitro- 
gen, and  equilibrium  between  the  nitrogen  contents  of 
the  atmosphere  and  of  the  sea  is  thus  maintained.  Hence 
we  need  not  fear  any  noteworthy  depletion  of  the  nitro- 
gen contents  of  the  air.  This  conclusion  is  in  accord  with 
the  fact  that  no  notable  accumulation  of  fixed  nitrogen 
appears  to  have  taken  place  in  the  solid  and  liquid  con- 
stituents of  the  earth. 

The  reader  may  remember  (compare  page  57)  that 
during  the  annual  cycle  of  vegetation  not  less  than  one- 
fiftieth  of  the  atmospheric  contents  in  the  carbon  dioxide 
is  absorbed.  Since  oxygen  is  formed  from  this  carbon 
dioxide,  and  since  the  air  contains  about  seven  hundred 
times  •  as  many  parts  per  volume  of  oxygen  as  of  carbon 
dioxide,  the  exchange  of  atmospheric  oxygen  is  about 
one  part  in  thirty-five  thousand.  In  other  words,  the 
oxygen  of  the  air  participates  about  one  hundred  times 
more  energetically  in  the  processes  of  vegetation  than  the 
nitrogen,  and  this  is  in  accordance  with  the  general  high 
chemical  activity  of  oxygen. 

Before  we  close  this  chapter  we  will  briefly  refer  to 
the  peculiar  phenomenon  known  as  the  Zodiacal  Light, 
which  can  be  seen  in  the  tropics  almost  any  clear  night 
for  a  few  hours  after  or  before  sunset  or  sunrise.  In  our 
latitudes  the  light  is  rarely  visible,  and  is  best  seen  about 
the  periods  of  the  vernal  and  autumnal  equinoxes.  The 
phenomenon  is  generally  described  as  a  luminous  cone 

145 


WORLDS   IN   THE    MAKING 

whose  basis  lies  on  the  horizon,  and  whose  middle  line 
coincides  with  the  zodiac.  Hence  the  name.  Accord- 
ing to  Wright  and  Liais,  its  spectrum  is  continuous.  It 
is  stated  that  the  Zodiacal  Light  is  as  strong  in  the  tropics 
as  that  of  the  Milky  Way. 

There  can  be  no  doubt  that  this  glow  is  due  to  dust 
particles  illuminated  by  the  sun.  It  has  therefore  been 
assumed  that  this  dust  is  floating  about  the  sun  in  a  ring, 


Fig.  44. — Zodiacal  Light  in  the  tropics 
146 


VARIATIONS  OF  TERRESTRIAL  MAGNETISM 

and  that  it  represents  the  rest  of  that  primeval  nebula 
out  of  which  the  solar  system  has  been  condensed,  ac- 
cording to  the  theory  of  Kant  and  Laplace  (compare 
Chapter  VII.).  Sometimes  a  fairly  luminous  band  seems 
to  shoot  out  from  the  apex  of  the  cone  of  the  Zodiacal  Light 
and  to  cross  the  sky  in  the  plane  of  the  ecliptic.  In  that 
part  of  the  sky  which  is  just  opposite  the  sun  this  band 
expands  to  a  larger,  diffused,  not  well-defined  spot  of 
light  covering  about  12°  of  arc  in  latitude  and  90°  in  lon- 
gitude. This  luminescence  is  called  the  counter -glow 
(Gegenschein),  and  was  first  described  by  Pezenas  in 
1780. 

The  most  probable  view  concerning  the  nature  of  this 
counter-glow  is  that  it  is  caused  by  small  particles  of 
meteorites  or  dust  which  fall  towards  the  sun.  Like  the 
position  of  the  corona  of  the  aurora,  the  position  of  this 
counter-glow  seems  to  be  an  effect  of  perspective;  the 
orbits  of  the  little  particles  are  directed  towards  the  sun, 
and  they  therefore  appear  to  radiate  from  a  point  op- 
posite to  it. 

We  know  very  little  about  this  phenomenon.  Even 
the  position  of  the  Zodiacal  Light  along  the  zodiac  which 
has  given  rise  to  its  name  has  been  questioned,  and  it 
would  appear  from  recent  investigations  that  the  glow 
is  situated  in  the  plane  of  the  solar  equator.  However 
that  may  be,  the  view  is  very  generally  held  that  the  glow 
is  due  to  particles  which  come  from  the  sun  or  enter 
into  it.  We  have  already  adduced  arguments  to  prove 
that  the  mass  of  solar  dust  cannot  be  unimportant;  this 
dust  may  therefore  be  the  cause  of  the  phenomenon 
which  we  have  just  been  discussing. 


VI 

END   OF   THE   SUN— ORIGIN   OF   NEBULA 

WE  have  seen  that  the  sun  is  dissipating  and  wasting 
almost  inconceivable  amounts  of  heat  every  year:  3.8. 
1033  gramme-calories,  corresponding  to  2  gramme-calories 
for  each  gramme  of  its  mass.  We  have  also  obtained  an 
idea  as  to  how  the  enormous  storage  of  heat  energy  in 
the  sun  may  endure  this  loss  for  ages.  Finally,  however, 
the  time  must  come  wken  the  sun  will  cool  down  and 
when  it  will  cover  itself  with  a  solid  crust,  as  the  earth 
and  the  other  planets  —  so  far,  probably,  in  a  gaseous 
state — have  done  long  since  or  will  do  before  long.  No 
living  being  will  be  able  to  watch  this  extinction  of  the 
sun  despairingly  from  one  of  the  wandering  planets; 
for,  in  spite  of  all  our  inventions,  all  life  will  long  before 
have  ceased  on  the  satellites  of  the  sun  for  want  of  heat 
and  light. 

The  further  development  of  the  cold  sun  will  recall  the 
actual  progress  of  our  earth,  except  in  so  far  as  the  sun 
will  have  no  life-spending,  central  source  of  light  and 
heat  near  it.  In  the  beginning  the  thin,  solid  crust  will 
again  and  again  be  burst  by  gases,  and  streams  of  lava 
will  rush  out  from  the  interior  of  the  sun.  After  a  while 
these  powerful  discharges  will  stop,  the  lava  will  freeze, 
and  the  fragments  will  close  up  more  firmly  than  before. 
Only  on  some  of  the  old  fissures  volcanoes  will  rise  and 
allow  the  gases  to  escape  from  the  interior — water  vapor 

148 


END  OF  THE   SUN 

and,  to  a  less  extent,  carbonic  acid,  liberated  by  the 
cooling. 

Then  water  will  be  condensed.  Oceans  will  flood  the 
sun,  and  for  a  short  period  it  will  resemble  the  earth  in  its 
present  condition,  though  with  the  one  important  differ- 
ence. The  extinct  sun,  unlike  our  earth,  will  not  receive 
life-giving  heat  from  the  outside,  excepting  the  small 
amount  of  radiation  from  universal  space  and  the  heat 
generated  by  the  fall  of  meteorites.  The  temperature  fall 
will  therefore  be  rapid,  and  the  vanishing  clouds  of  the  at- 
tenuated atmosphere  will  not  long  check  radiation.  The 
ocean  will  become  covered  with  a  crust  of  ice.  Then  the 
carbonic  acid  will  commence  to  condense,  and  will  be  pre- 
cipitated as  a  light  snow  in  the  solar  atmosphere.  Fi- 
nally, at  a  temperature  of  about  —200° Cent.,  the  gases  of 
the  atmosphere  will  be  condensed,  and  new  oceans,  now 
principally  of  nitrogen,  will  be  produced.  Let  the  tem- 
perature sink  another  20°,  and  the  energy  of  the  inrush- 
ing  meteorites  will  just  suffice  to  balance  a  further  loss 
of  heat  by  radiation.  The  solar  atmosphere  will  then 
consist  •  essentially  of  helium  and  hydrogen  —  the  two 
gases  which  are  most  difficult  to  condense — and  of  some 
nitrogen. 

In  this  stage  the  heat  loss  of  the  sun  will  be  almost 
imperceptible.  Owing  to  the  low  thermal  conductive 
power  of  the  earth's  crust,  there  escapes  through  each 
square  mile  of  this  crust  scarcely  one-thousand-millionth 
part  of  the  heat  which  the  sun  is  radiating  from  an  equal 
area  of  its  surface.  In  future  days,  when  the  solar  crust 
will  have  attained  a  thickness  of  60  km.  (40  miles),  its 
loss  of  heat  will  be  diminished  to  the  same  degree.  The 
temperature  on  the  surface  of  the  sun  may  then  still  be 
some  50°  or  60°  above  absolute  zero,  and  volcanic  erup- 
tions will  raise  the  temperature  only  for  short  periods 

149 


WORLDS    IN   THE   MAKING 

and  over  small  areas.  Yet  in  the  interior  the  tempera- 
ture will  still  be  at  nearly  the  same  actual  intensity,  some- 
thing like  several  million  degrees,  and  the  compounds  of 
infinite  explosive  energy  will  be  stored  up  there  as  to- 
day. Like  an  immense  dynamite  magazine,  the  dark  sun 
will  float  about  in  universal  space  without  wasting  much 
of  its  energy  in  the  course  of  billions  of  years.  Immutable, 
like  a  spore,  it  will  retain  its  immense  store  of  force  until 
it  is  awakened  by  external  forces  into  a  new  span  of  life 
similar  to  the  old  life.  A  slow  shrinkage  of  the  surface, 
due  to  the  progressive  loss  of  heat  of  the  core  and  to  the 
consequent  contraction,  will  in  the  meanwhile  have  cov- 
ered the  sun  with  the  wrinkles  of  old  age. 

Let  us  suppose  that  the  crusts  of  the  sun  and  the  earth 
have  the  same  thermal  conductivity — namely,  that  of 
granite.  According  to  Homen,  a  slab  of  granite  one 
centimetre  in  thickness,  whose  two  surfaces  are  at  a  tem- 
perature difference  of  1°  Cent.,  will  permit  0.582  calorie 
to  pass  per  minute  per  square  centimetre  of  surface. 
By  analogy,  the  earth's  crust,  with  an  increase  of  tem- 
perature of  30°  per  kilometre,  as  we  penetrate  inward, 
would  allow  1.75  .10~4  calorie  to  pass  per  minute  and 
per  square  centimetre  (this  is  -g-gVo  °f  the  mean  heat  sup- 
ply of  the  earth,  0.625  calorie  per  minute  per  square  centi- 
metre); while  the  sun,  with  a  crust  of  the  same  thick- 
ness as  the  earth,  but  with  a  diameter  108.6  times  larger, 
would  lose  3.3  times  more  heat  per  minute  than  the  earth 
receives  from  it  at  the  present  time.  At  present  the  sun 
loses  2260  million  times  more  heat  than  the  earth  re- 
ceives; consequently,  the  loss  of  heat  would  be  reduced 
t°  6T67ffo0T<ra~(T  °f  the  present  amount.  If  the  thickness 
of  the  solar  crust  amounted  to  yj-^  of  the  solar  radius — 
that  is  to  say,  to  the  same  fraction  that  the  thickness  of 
the  earth's  crust  represents  of  the  terrestrial  radius — the 

150 


END  OF  THE  SUN 

sun  would  in  74,500  million  years  not  lose  any  more  heat 
than  it  does  now  in  a  single  year.  This  number  has  to 
be  diminished,  on  account  of  the  colder  surface  which 
the  sun  would  have  by  that  time,  to  about  60,000  million 
years.  Considering  that  the  mean  temperature  of  the 
sun  may  be  as  high  as  5  million  degrees  Celsius,  the  cool- 
ing down  to  the  freezing-point  of  water  might  occupy 
150,000  billion  years,  assuming  that  its  mean  specific 
heat  is  as  great  as  that  of  water.  During  this  time  the 
crust  of  the  sun  would  increase  in  thickness  and  the  cool- 
ing would,  of  course,  proceed  at  a  decreasing  rate.  In 
any  case,  the  total  loss  of  energy  during  a  period  of  a 
thousand  billion  years  could,  under  these  circumstances, 
only  constitute  a  very  small  fraction  of  the  total  stored 
energy. 

When  an  extinct  star  moves  forward  through  infinite 
spaces  of  time,  it  will  ultimately  meet  another  luminous 
or  likewise  extinct  star.  The  probability  of  such  a  col- 
lision is  proportional  to  the  angle  under  which  the  star  ap- 
pears— which,  though  very  small,  is  not  of  zero  magnitude 
—and  to  the  velocity  of  the  sun.  The  probability  is  in- 
creased by  the  deflection  which  these  celestial  bodies  will 
undergo  in  their  orbits  on  approaching  each  other.  Our 
nearest  neighbors  in  the  stellar  universe  are  so  far  removed 
from  us  that  light,  the  light  of  our  sun,  requires,  on  an  av- 
erage, perhaps  ten  years  to  reach  them.  In  order  that  the 
sun,  with  its  actual  dimensions  and  its  actual  velocity  in 
space  —20  km.  (13  miles)  per  second — should  collide  with 
another  star  of  similar  kind,  we  should  require  something 
like  a  hundred  thousand  billion  years.  Suppose  that 
there  are  a  hundred  times  more  extinct  than  luminous 
stars — an  assumption  which  is  not  unjustifiable — the 
probable  interval  up  to  the  next  collision  may  be  some- 
thing like  a  thousand  billion  years.  The  time  during 

ii  151 


WORLDS  IN   THE   MAKING 

which  the  sun  would  be  luminous  would  represent  per- 
haps one-hundredth  of  this — that  is  to  say,  ten  billion 
years.  This  conclusion  does  not  look  unreasonable.  For 
life  has  only  been  existing  on  the  earth  for  about  a  thou- 
sand million  years,  and  this  age  represents  only  a  small 
fraction  of  the  time  during  which  the  sun  has  emitted 
light  and  will  continue  to  emit  light.  The  probability  of 
a  collision  between  the  sun  and  a  nebula  is,  of  course, 
much  greater;  for  the  nebulae  extend  over  very  large 
spaces.  In  such  a  case,  however,  we  need  not  apprehend 
any  more  serious  consequences  than  result  when  a  comet 
is  passing  through  the  corona  of  the  sun.  Owing  to  the 
very  small  amount  of  matter  in  the  corona,  we  have  not 
perceived  any  noteworthy  effects  in  these  instances. 
Nevertheless,  the  entrance  of  the  sun  into  a  nebula  would 
increase  the  chance  of  a  collision  with  another  sun;  for 
we  shall  see  below  that  dark  and  luminous  celestial 
bodies  appear  to  be  aggregated  in  the  nebulae. 

From  time  to  time  we  see  new  stars  suddenly  flash  up 
in  the  sky,  rapidly  decrease  in  splendor  again,  to  become 
extinguished  or,  at  any  rate,  to  dwindle  down  to  faint 
visibility  once  more.  The  most  remarkable  of  these  ex- 
ceedingly interesting  events  occurred  in  February,  1901, 
when  a  star  of  the  first  magnitude  appeared  in  the  con- 
stellation of  Perseus.  This  star  was  discovered  by  An- 
derson, a  Scotchman,  on  the  morning  of  February  22, 
1901.  It  was  then  a  star  of  the  third  magnitude.1  On 
a  photograph  which  had  been  taken  only  twenty-eight 


1  Stars  are  classified  in  magnitude,  the  order  being  such  that  the 
most  luminous  stars  have  the  lowest  numbers.  A  star  of  the  first 
magnitude  is  2.52  times  brighter  than  a  star  of  the  second  magnitude; 
this,  again,  2.52  times  brighter  than  a  star  of  the  third  magnitude, 
and  so  on.  All  this  from  the  point  of  view  of  an  observer  on  the 
earth. 

152 


ORIGIN   OF  NEBULAE 

hours  previous  to  the  discovery  of  this  star,  the  star  was 
not  visible  at  all,  although  the  plate  marked  stars  down 
to  the  twelfth  magnitude.  The  light  intensity  of  this 
new  star  would  hence  appear  to  have  increased  more 
than  five-thousand-fold  within  that  short  space  of  time. 
On  February  23d  the  new  star  surpassed  all  other  stars 
except  Sirius  in  intensity.  By  February  25th  it  was  of 
the  first  magnitude,  by  February  27th  of  the  second,  by 
March  6th  of  the  third,  and  by  March  18th  of  the  fourth 
magnitude.  Then  its  brightness  began  to  fluctuate  peri- 
odically up  to  June  22d,  with  a  period  first  of  three,  then 
of  five  days,  while  the  average  light  intensity  decreased. 
By  June  23d  it  was  of  the  sixth  magnitude.  The  light 
intensity  diminished  then  more  uniformly.  By  October, 
1901,  it  was  a  star  of  the  seventh  magnitude ;  by  February, 
1901,  of  the  eighth  magnitude;  by  July,  1902,  of  the 
ninth  magnitude;  by  December,  1902,  of  the  tenth  mag- 
nitude; and  since  then  it  has  gradually  dwindled  to  the 
twelfth  magnitude.  When  this  star  was  at  its  highest 
intensity  it  shone  with  a  bluish- white  light.  The  shade 
then  changed  into  yellow,  and  by  the  beginning  of  March, 
1901,  into  reddish.  During  its  periodical  fluctuations  the 
hue  was  whitish  yellow  at  its  maximum  and  reddish  at 
its  minimum  intensity.  Since  then  the  color  has  grad- 
ually passed  into  pure  white. 

The  spectrum  of  this  star  shows  the  greatest  similarity 
to  that  of  the  new  star  in  the  constellation  Auriga  (Nova 
Aurigae)  of  the  year  1892  (Fig.  45). 

On  the  whole,  it  is  characteristic  of  new  stars  that  the 
spectrum  lines  appear  double — dark  on  the  violet  arid 
bright  on  the  red  side.  In  the  spectrum  of  Nova  Aurigse 
this  peculiarity  is,  among  others,  striking  in  the  three  hy- 
drogen lines  C,  F,  and  H,  in  the  sodium  line,  in  the  nebula 

lines,  and  also  in  the  magnesium  line.     In  the  spectrum 

153 


WORLDS  IN  THE  MAKING 


CJ» 

CSO  (60  GM) 


Fig.  45. — Spectrum  of  Nova  Aurigae,  1892 

of  Nova  Persei  the  displacement  of  the  hydrogen  lines 
towards  the  violet  is  so  great  that,  according  to  Doppler's 
principle,1  the  hydrogen  gas  which  absorbed  the  light 
must  have  been  moving  towards  us  with  a  velocity  of 
70Q  or  more  kilometres  (450  miles)  per  second.  Some 
calcium  lines  show  a  similar  displacement,  which  is  less 
noticeable  in  the  case  of  the  other  metals.  This  would 

1  When,  standing  on  a  station  platform,  we  watch  an  express  train 
rushing  through  the  station,  the  pitch  of  the  engine  whistle  seems  to 
become  higher  as  long  as  the  train  is  approaching  us,  and  deeper  again 
when  the  train  is  moving  away  from  us.  The  pitch  of  a  note  depends 
upon  the  number  of  oscillations  which  our  ear  receives  per  second. 
Now,  when  the  train  is  fast  approaching  us,  more  vibrations  are  sent 
into  our  ear  than  when  the  train  is  at  a  stand-still,  and  the  pitch, 
therefore,  appears  to  become  higher.  The  same  reasoning  holds  for 
light  waves,  of  which  Doppler,  of  Prague  (Bohemia),  was  in  fact  think- 
ing when  first  announcing  his  principle  in  1842.  The  wave-length  of 
a  particular  color  of  the  spectrum  is  fixed  with  the  aid  of  some  Fraun- 
hofer  line  characteristic  of  a  certain  metal.  If  we  compare  the  spec- 
trum of  a  star  and  the  spectrum  of  a  glowing  metal,  photographed 
on  the  same  plate,  the  stellar  lines  will  appear  shifted  towards  the 
violet  end  (violet  light  is  produced  by  nearly  twice  as  many  vibrations 
of  the  ether  per  second  as  red  light)  when  the  star  is  moving  towards 
us  in  the  line  of  sight.  This  principle  has  successfully  been  applied 
by  Huggins,  H.  C.  Vogel,  and  others,  for  determining  the  motion  of  a 
star  in  our  line  of  sight.  When  a  star  is  revolving  about  its  own 
axis,  the  equatorial  belt  will  seem  to  come  nearer  to  us  (or  to  recede 
from  us),  while  the  polar  regions  will  seem  to  be  at  a  stand-still;  the 
lines  will  then  appear  oblique  (not  vertical).  In  this  way  Keeler 
proved  that  the  rings  of  Saturn  consists  of  swarms  of  meteorites  mov- 
ing at  different  velocities  in  the  different  rings. — H.  B. 

154 


ORIGIN  OF  NEBULA 

appear  to  indicate  that  relatively  cold  masses  of  gas  are 
issuing  from  the  stars  and  streaming  with  enormous " 
velocities  towards  the  earth.  The  luminous  parts  of  the 
stars  wrere  either  at  a  stand-still  or  they  were  moving  away 
from  us.  The  simplest  explanation  of  these  phenomena 
would  be  that  a  star  when  flashing  up  by  virtue  of  its 
high  temperature  and  high  pressure  shows  enhanced 
(widened)  spectral  lines,  whose  violet  portion  is  absorbed 
by  the  strongly  cooled  masses  of  gas  which  are  moving 
towards  us  and  are  cooled  by  their  own  strong  expansion. 
These  gases  stream,  of  course,  in  all  directions  from  the 
star,  but  we  only  become  aware  of  those  gases  which  ab- 
sorb the  light  of  the  stars — that  is  to  say,  those  situated 
between  the  star  and  the  earth,  and  streaming  in  our  di- 
rection. 

Gradually  the  light  of  the  metallic  lines  and  of  the  con- 
tinuous spectrum  on  which  they  were  superposed  began 
to  fade,  first  in  the  violet,  while  the  hydrogen  lines  and 
nebular  lines  still  remained  distinct ;  like  other  new  stars, 
this  star  showed,  after  a  while,  the  nebular  spectrum. 
This  interesting  fact  was  first  noticed  by  H.  C.  Vogel  in 
the  new  star  in  the  Swan  (Nova  Cygni,  1876).  The  star 
P  in  the  Swan,  which  flashed  up  in  the  year  1600,  still 
offers  us  a  spectrum  which  indicates  the  emission  of  hydro- 
gen gas.  It  is  not  impossible  that  this  "new"  star  has 
not  yet  reached  its  equilibrium,  and  is  still  continuing  to 
emit  cold  streams  of  gases.  Insignificant  quantities  of 
gas  suffice  for  the  formation  of  an  absorption  spectrum; 
thus  the  emission  of  gas  might  continue  for  long  periods 
without  exhausting  the  supply. 

We  have  already  mentioned  (page  116)  the  peculiar 
clouds  of  light  which  were  observed  around  Nova  Persei. 
Two  annular  clouds  moved  away  from  this  star  with  veloc- 
ities of  1.4  and  2.8  seconds  of  arc  per  day  between  March 

155 


WORLDS   IN   THE   MAKING 

29,  1901,  and  February,  1902.  If  we  calculate  backward 
from  these  dates  the  time  which  must  have  elapsed  since 
those  gases  left  the  star,  we  arrive  at  the  date  of  the 
week — February  8  to  16,  1901 — in  satisfactory  agreement 
with  the  period  of  greatest  luminosity  of  the  star  of 
February  23d.  It  would,  therefore,  appear  that  these 
emanations  came  from  the  star  and  were  ejected  by  the 
radiation  pressure.  Their  light  did  not  mark  any  no- 
ticeable polarization,  and  could  not  be  reflected  light  for 
this  reason.  We  may  suppose  that  the  dust  particles 
discharged  their  electric  charges,  and  that  the  gases  be- 
came thereby  luminous. 

In  this  case  we  were  evidently  witnesses  of  the  grand 
finale  of  the  independent  existence  of  a  celestial  body 
by  collision  with  some  other  body  of  equal  kind.  The 
two  colliding  bodies  were  both  dark,  or  they  emitted  so 
little  light  that  their  combined  light  intensities  did  not 
equal  that  of  a  star  of  the  twelfth  magnitude.  As,  now, 
their  splendor  after  the  collision  was  greater  than  that 
of  a  star  of  the  first  magnitude,  although  their  distance 
has  been  estimated  to  be  at  least  120  light  years,1  their 
radiation  intensity  must  have  exceeded  that  of  the  sun 
several  thousand  times.  Under  these  circumstances  the 
mechanical  radiation  pressure  must  also  have  been  many 
times  more  powerful  than  on  the  surface  of  the  sun,  and 
the  masses  of  dust  which  were  ejected  by  the  new  star 
must  have  possessed  a  velocity  very  much  greater  than 
that  of  solar  dust.  Yet  this  velocity  i$ust  have  been 
smaller  than  that  of  light,  which,  indeed,  the  effect  of 
the  radiation  pressure  can  never  equal. 

It  is  not  difficult  to  picture  to  ourselves  the  enormous 
violence  with  which  this  " collision"  must  have  taken 

1  One  light  year  corresponds  to  9.5  billions  kilometres,  and  it  is 
the  distance  which  the  light  traverses  in  the  course  of  a  year. 

156 


ORIGIN  OF   NEBULA 

place.  A  strange  body  —  for  instance,  a  meteorite— 
which  rushes  from  the  infinite  universe  into  the  sun  has 
at  its  collision  a  velocity  of  600  km.  (400  miles)  per 
second,  and  the  velocity  of  the  two  colliding  suns  must 
have  been  of  approximately  that  order.  The  impact 
will  in  general  be  oblique,  and,  although  part  of  the 
energy  will  of  course  be  transformed  into  heat,  the  rest 


Fig.  46. — Diagram  indicating  the  consequences  of  a  collision 
between  two  extinct  suns,  A  and  B  '  moving '  in  the  direc- 
tion of  the  straight  arrows.  A  rapid  rotation  in  the  di- 
rection of  the  curved  arrows  results,  and  two  powerful 
streamers  are  ejected  by  A  B,  the  explosive  substances  from 
the  deeper  strata  of  A  and  B  being  brought  up  to  the  sur- 
face by  the  collision 

of  the  kinetic  energy  must  have  produced  a  rotational 
velocity  of  hundreds  of  kilometres  per  second.  By  com- 
parison with  this  number  the  actual  circumferential  speed 
of  the  sun,  about  2  km.  (1}  miles)  per  second  on  the 
equator,  would  vanish  altogether;  and  the  difference  is 
still  more  striking  for  the  earth,  with  its  0.465  km.  per 
second  at  the  equator.  We  shall,  therefore,  not  commit 

an  error  of  any  consequence  if  we  presume  the  two  bodies 

157 


WORLDS    IN   THE   MAKING 

to  have  been  practically  devoid  of  circumferential  speeds 
before  their  collision.  At  the  collision,  matter  will  have 
been  ejected  from  both  these  celestial  bodies  at  right 
angles  to  the  relative  directions  of  their  motion  in  two 
powerful  torrents,  which  would  be  situated  in  the  plane 
in  which  the  two  bodies  were  approaching  each  other 
(compare  Fig.  46).  The  rotational  speed  of  the  double 
star,  which  will  be  diminished  by  this  ejection  of  matter, 
will  have  contributed  to  increase  the  energy  of  ejection. 
We  remember,  now,  that  when  matter  is  brought  up 
from  the  interior  to  the  surface  of  the  sun  it  will  behave 
like  an  explosive  of  enormous  power.  The  ejected  gases 
will  be  hurled  in  terrific  flight  about  the  rapidly  revolving 
central  portions.  We  obtain  an  idea  (though  a  very  im- 
perfect one)  of  these  features  when  we  look  at  a  revolving 
pinwheel  in  a  fireworks  display.  Two  pinwheels  have 
been  attached  to  the  ends  of  a  diameter  and  belch  forth 
fire  in  radial  lines.  The  farther  removed  from  the  wheel, 
the  smaller  will  be  the  actual  velocity  and  also  the  angu- 
lar velocity  of  these  torrents  of  fire.  Similarly  with 
the  star.  The  streams  are  rapidly  cooled,  owing  to  the 
rapid  expansion  of  the  gas.  They  will  also  contain  fine 
dust,  largely  consisting  of  carbon,  probably,  which  had 
been  bound  by  the  explosive  materials.  The  clouds  of 
fine  dust  will  obscure  the  new  star  more  and  more,  and 
will  gradually  change  its  white  brilliancy  into  yellow  and 
reddish,  because  the  fine  dust  weakens  blue-and-green 
rays  more  than  it  does  yellow-and-red  rays.  At  first  the 
clouds  were  so  near  to  the  the  star  that  they  possessed  a 
high  angular  velocity  of  their  own;  they  then  appeared  to 
surround  the  star  completely.  But  after  March  22,  1901, 
the  outer  particles  of  the  streams  attained  greater  dis- 
tances and  assumed  longer  periods  of  revolution  (six 
days);  the  star  then  became  more  obscured  when  the 

158 


ORIGIN  OF  NEBULAE 

extreme  dust  clouds  of  the  streams  covering  it  happened 
to  get  between  us  and  the  star.  As  the  streams  of  par- 
ticles were  moving  farther  away,  their  rotational  periods 
increased  gradually  to  ten  days.  The  star,  therefore, 
became  periodical  with  a  slowly  growing  length  of 
period,  and  its  glow  turned  more  reddish  at  its  min- 
imum than  at  its  maximum  of  intensity.  At  the  same 
time,  the  absorptive  power  of  the  marginal  particles 
decreased,  partly  owing  to  their  increasing  expansion, 
partly  because  the  dust  was  slowly  aggregating  to  coarser 


Fig.  47. — Spiral  nebula  in  the  Canes  Venatici.  Messier  51. 
Taken  at  the  Yerkes  Observatory  on  June  3,  1902.  Scale, 
1  mm. =13. 2  sec.  of  arc 

159 


WORLDS    IN   THE   MAKING 

particles ;  possibly,  also,  because  the  finest  particles  were 
being  driven  away  by  the  radiation  pressure.  The  sift- 
ing influence  which  the  dust  exercised  upon  the  light, 
and  owing  to  which  the  red-and-yellow  rays  were  more 
readily  transmitted  than  the  blue-and-green,  gradually 
became  impaired ;  hence  the  color  of  the  light  Burned 
more  gray,  and  after  a  certain  time  the  star  appeared 
once  more  of  a  whitish  hue.  This  white  color  indicates 
that  the  star  must  still  have  a  very  high  temperature. 
By  the  continued  ejection  of  dust-charged  masses  of 
gas,  probably  with  gradually  decreasing  violence,  the 
light  intensity  of  the  star  must  slowly  diminish  (as  seen 
from  the  earth)  and  the  distribution  of  the  layers  of 
dust  around  the  luminous  core  will  more  and  more  be- 
come uniform.  How  violent  the  explosion  must  have 
been,  we  recognize  from  the  observation  that  the  first 
ejected  masses  of  hydrogen  rushed  out  with  an  apparent 
velocity  of  at  least  700  km.  per  second.  This  velocity  is 
of  the  same  order  as  that  of  the  most  remarkable  promi- 
nences of  the  sun. 

It  will  be  admitted  that  these  arguments  present  us 
with  a  faithful  simile  even  of  the  details  of  the  observed 
course  of  events,  and  it  is  therefore  highly  probable  that 
our  view  is  in  the  main  correct.  But  what  has  mean- 
while become  of  the  new  star?  Spectrum  analysis  tells  us 
that  it  has  been  converted  into  a  stellar  nebula  like  other 
new  stars.  The  continuous  light  of  the  central  body  has 
more  and  more  been  weakened  by  the  surrounding  masses 
of  dust.  By  the  radiation  pressure  these  masses  are 
driven  towards  the  outer  particles  of  the  surrounding 
gaseous  envelope  consisting  principally  of  hydrogen, 
helium,  and  "  nebular  matter."  There  the  dust  dis- 
charges its  negative  electricity,  and  thus  calls  forth  a 
luminescence  which  equals  that  of  the  nebulae. 

160 


Fig.  48.— Spiral  nebula  in  the  Triangle.  Messier  33.  Taken  at  the 
Yerkes  Observatory  on  September  4  and  6,  1902.  Scale,  1  nun.= 
30.7  sec.  of  arc 


WORLDS   IN   THE   MAKING 

We  have  to  consider  next  that  owing  to  the  incredibly 
rapid  rotation,  the  central  main  mass  of  the  two  stars 
will,  in  its  outer  portions,  be  exposed  to  centrifugal  forces 
of  extraordinary  intensity,  and  will  therefore  become 
flattened  out  to  a  large  revolving  disk.1  As  the  pressure 
in  the  outer  portions  will  be  relatively  small,  the  density 
of  the  gases  will  likewise  be  diminished  there.  The  en- 
ergetic expansion  and,  more  still,  the  great  heat  radiation 
will  lower  the  temperature  at  a  rapid  rate.  We  have 
thus  to  deal  with  a  central  body  whose  inner  portion  will 
possess  a  high  density,  and  which  will  resemble  the  mass 
of  the  sun,  while  the  outer  portion  will  be  attenuated  and 
nebular.  Distributed  about  the  central  body  we  shall 
find  the  rest  of  the  two  streams  of  gases  which  were  eject- 
ed immediately  after  the  violent  collision  between  the 
two  celestial  bodies.  A  not  inconsiderable  portion  of 
the  matter  of  these  spirally  arranged  outer  parts  will 
probably  travel  farther  away  into  infinite  space,  finally 
to  join  some  other  celestial  body  or  to  form  parts  of  the 
great  irregular  spots  of  nebular  matter  which  are  col- 
lected around  the  star  clusters.  Another  portion,  not 
able  to  leave  the  central  body,  will  remain  in  circular 
movement  about  it.  In  consequence  of  this  circular 
movement,  which  will  be  extremely  slow,  the  outlines  of 
the  two  spirals  will  gradually  become  obliterated,  and 
the  spirals  will  themselves  more  and  more  assume  the 
shape  of  nebular  rings  about  the  central  mass. 

This  spiral  form  (Figs.  47  and  48)  of  the  outer  portions 


1  A.  Hitter  has  calculated  that  when  two  suns  of  equal  size  collide 
with  one  another  from  an  infinite  distance,  the  energy  of  the  collision 
is  not  more  than  sufficient  to  enlarge  the  volume  of  the  suns  to  four 
times  the  previous  amount.  The  largest  portion  of  the  mass  will 
therefore  probably  remain  in  the  centre,  and  it  will 'only  be  masses 
of  light  gases  which  will  be  ejected. 

162 


Fig.  49.— The  great   nebula    in  Andromeda.     Taken  at  the  Yerkes 
Observatory  on  September  18, 1901.     Scale,  1  mm.  =  54.6  sec.  of  arc 


WORLDS    IN    THE   MAKING 

of  the  nebulse  has  for  a  long  time  excited  the  greatest 
attention.  In  almost  all  the  investigated  instances  it 
has  been  observed  that  two  spiral  branches  are  coiling 
about  the  central  body.  This  would  indicate  that  the 
matter  is  in  a  revolving  movement  about  the  cenjtral  axis 
of  the  spiral,  and  that  it  has  streamed  away  from  the  axis 
in  two  opposite  directions.  Sometimes  the  matter  ap- 
pears arranged  as  in  a  coil;  of  this  type  the  great  nebula 
of  Andromeda  is  the  best-known  example  (Fig.  49).  A 
closer  inspection  of  this  nebula  with  more  powerful  in- 
struments indicates,  however,  that  it  is  also  spiral  and 
that  it  appears  coiled,  because  we  are  looking  at  it  from 


Fig.  50. — Ring-shaped  nebula  in  Lyra.     Taken  at 
the  Yerkes  Observatory 

the  side.  The  late  famous  American  astronomer  Keeler, 
who  has  studied  these  nebulae  with  greater  success  than 
any  one  else,  has  catalogued  a  great  many  of  them  in  all 
the  divisions  of  the  heavens  which  were  accessible  to  his 

164 


ORIGIN  OF  NEBULA 

instruments,  and  he  has  found  that  these  formations  are 
predominatingly  of  a  spiral  nature. 

Some   nebulae,   like   the   so-called   planetary   nebulae, 
offer  rather  the  appearance  of  luminous  spheres.     We 


Fig.  51. — Central  portion  of  the  great  nebula  in  Orion.     Taken  at 
the  Yerkes  Observatory.      Scale,  1  mm. =12  sec.  of  arc 

may  assume  in  these  cases  that  the  explosions  were  less 
violent,  and  that  the  spirals,  therefore,  are  situated  so 
closely  together  that  they  seem  to  merge  into  one  an- 
other. Possibly  the  inequalities  in  their  development 
have  become  equalized  in  the  course  of  time.  A  few 
nebuke  are  ring-shaped,  as  the  well-known  nebula  of 
Lyra  (see  Fig.  50).  These  rings  may,  again,  have  been 

165 


WORLDS   IN    THE   MAKING 

formed  out  of  spiral  nebulse,  and  the  spirals  may  have 
gradually  been  obliterated  by  rotation,  while  the  central 
nebulous  matter  may  have  been  concentrated  on  the 
planets  travelling  round  the  central  star.  Schaeberle,  an 
eminent  American  astronomer,  has  discovered  traces  of 
spiral  shape  also  in  the  Lyra  nebula. 

Another  kind  of  nebula  is  the  ordinary  nebula  of 
vast  extension  and  irregular  shape,  evidently  formed  out 
of  most  extremely  attenuated  matter;  well-known  char- 
acteristic examples  are  found  in  Orion,  about  the  Pleiades, 
and  in  the  Swan  (Figs.  51,  52,  and  53).  In  these  nebula? 
portions  of  a  spiral  structure  have  likewise  often  been  dis- 
cerned. 

We  have  said  that  the  collision  between  two  celestial 
bodies  would  result  in  the  formation  of  a  spiral  with  two 
wings.  If  the  impact  is  such  that  the  two  centres  of  the 
celestial  bodies  move  straight  towards  each  other,  a  disk 
will  arise,  and  not  a  spiral ;  or  if  one  star  is  much  smaller 
than  the  other,  possibly  a  cone,  because  the  gases  will 
uniformly  be  spread  in  all  directions  about  the  line  of 
impact.  A  perfectly  central  impact  is  obviously  very 
rare;  but  there  may  be  cases  which  approach  this  limit- 
ing condition  more  or  less,  especially  when  the  relative 
velocity  of  the  two  bodies  is  small.  By  slow  diffusion 
a  feebly  developed  spiral  may  also  be  converted  into  a 
rotating  disklike  structure.  The  extension  of  these 
nebular  structures  will  depend  upon  the  ratio  between 
the  mass  of  the  system  and  the  velocity  of  ejection  of 
the  gases.  If,  for  example,  two  extinct  suns  of  nearly 
equal  dimensions  and  mass,  like  our  sun,  should  collide, 
some  gas  masses  would  travel  into  infinite  space,  being 
hurled  out  with  a  velocity  of  more  than  900  km.  (550 
miles)  per  second;  while  other  particles,  moving  at  a 
slower  rate,  would  remain  in  the  neighborhood  of  the 

166 


Fig.  52.— Nebular  striae  in  the  stars  of  the  Pleiades.  Taken  at  the 
Yerkes  Observatory  on  October  19,  1901.  Scale,  1  mm.  =42.2  sec. 
of  arc 


WORLDS   IN    THE   MAKING 

central  body.  The  nearer  to  that  body,  the  smaller 
was  their  velocity.  From  their  position  they  might  fall 
back  into  the  central  body,  to  be  reiucorporated  in  it, 
if  two  circumstances  did  not  prevent  this.  The  one  cir- 
cumstance is  the  enormous  radiation  pressure  of  the 
glowing  central  mass.  That  pressure  keeps  masses  of 
dust  particles  floating,  which  by  friction  will  carry  the 
surrounding  masses  of  gas  with  them.  Owing  to  the 
absorption  of  the  radiation  by  the  dust  particles,  only 
the  finer  particles  will  be  supported  farther  outside,  and 
at  the  extreme  margin  of  the  nebula  even  the  very  finest 
dust  will  no  longer  be  maintained  in  suspension  by  the 
greatly  weakened  radiation  pressure.  Thus  we  arrive  at 
an  outer  limit  for  the  nebula.  The  other  circumstance  is 
the  violent  rotation  which  is  set  up  by  the  impact  of  the 
central  bodies.  The  rotation  and  the  centrifugal  forces 
will  produce  a  disk-shaped  expansion  of  the  whole  cen- 
tral mass.  Owing  to  molecular  collisions  and  to  tidal 
effects,  the  angular  velocity  will  in  the  denser  portions 
tend  to  become  uniform,  so  that  the  whole  will  rotate 
like  a  flattened-out  ball  filled  with  gas,  and  the  spiral 
structure  will  gradually  disappear  in  those  parts.  In 
the  more  remote  particles  the  velocity  will  only  increase 
to  such  an  extent  as  to  equal  that  of  a  planet  moving 
at  the  same  distance — that  is  to  say,  the  gravitation  tow- 
ards the  central  body  will  be  balanced  by  the  centrifugal 
force,  and  at  the  very  greatest  distances  the  molecular 
bombardments,  as  well  as  gravitation  towards  the  centre, 
will  become  so  insignificant  that  any  masses  collected 
there  will  retain  their  shape  for  an  almost  unlimited  space 
of  time. 

In  the  centre  of  this  system  the  main  bulk  of  the 
matter  would  be  concentrated  in  a  sun  of  extreme 
brightness,  whose  light  intensity  would,  however,  owing 

168 


Fig.  53.— Nebular  striae  in  the  Swan.  New  General  Catalogue,  6992. 
Taken  at  the  Yerkes  Observatory  on  October  5, 1901.  Scale,  1  mm. 
=41  sec.  of  arc 


WORLDS  IN  THE   MAKING 

to  strong  radiation,  diminish  with   comparative  rapid- 
ity. 

Such  an  extensive  nebular  system,  in  which  gravitation, 
on  account  of  the  enormous  distances,  would  act  feebly 
and  very  slowly,  would  yet,  in  spite  of  the  extraordinary 
attenuation  of  matter  in  its  outer  portions,  and  just  on 
account  of  its  vast  extension,  be  able  to  stop  the  move- 
ment of  the  particles  of  dust  penetrating  into  it.  If  the 
gases  of  the  nebula  are  not  to  escape  into  space,  notwith- 
standing the  infinitesimal  gravitation,  their  molecules 
must  be  assumed  to  be  almost  at  a  stand-still,  and  their 
temperature  must  not  rise  by  more  than  50°  or  60°  Cent, 
above  absolute  zero.  At  such  low  temperatures  the  so- 
called  adsorption  plays  an  enormously  important  part 
(Dewar).  The  small  dust  particles  form  centres  about 
which  the  gases  are  condensed  to  a  remarkable  degree. 
The  extremely  low  density  of  these  gases  does  not  pre- 
vent their  condensation ;  for  the  adsorption  phenomenon 
follows  a  law  according  to  which  the  mass  of  condensed 
gas  will  only  be  reduced  by  about  one-tenth  when  the 
density  of  the  surrounding  gas  has  been  decreased  by 
one-ten-thousandth.  The  mass  of  dust  particles  or  dust 
grains  will  thus  be  augmented,  and  when  they  collide 
they  will  be  cemented  together  by  the  semi-liquid  films 
condensed  upon  them.  There  must,  hence,  be  a  rela- 
tively energetic  formation  of  meteorites  in  the  nebula?, 
and  especially  in  their  interiors.  Then  stars  and  their 
satellites,  migrating  through  space,  will  stray  into  these 
swarms  of  gases  and  meteorites  within  the  nebulae.  The 
larger  and  more  rapidly  moving  celestial  bodies  will 
crush  through  this  relatively  less  dense  matter;  but 
thousands  of  years  may  yet  be  occupied  in  their  passing 
through  nebulae  of  vast  dimensions. 

An  extraordinarily  interesting  photograph  obtained  by 

170 


a 


s 


I 


WORLDS    IN   THE    MAKING 

the  celebrated  Professor  Max  Wolf,  of  Heidelberg,  shows 
us  a  part  of  the  nebula  in  the  Swan  into  which  a  star  has 
penetrated  from  outside.  The  intruder  has  collected 
about  it  the  nebulous  matter  it  met  on  its  way,  and  has 
thus  left  an  empty  channel  behind  it  marking  its  track. 
Similar  spots  of  vast  extent,  relatively  devoid  of  nebulous 
matter,  occur  very  frequently  in  the  irregular  nebulse, 
they  are  frequently  called  "fissures,"  or  by  the  specifi- 


Fig.  55. — Great  nebula  near  Rho,  in  Ophiuchus.  Photograph  by 
E.  E.  Barnard,  Lick  Observatory.  There  are  several  empty  spots 
and  rifts  near  the  larger  stars  of  the  nebula 

172 


ORIGIN   OF  NEBULA 

cally  English  term  "rifts,"  because  they  have  generally 
a  long-drawn-out  appearance.  The  presumption  that 
these  rit'ts  represent  the  tracks  of  large  celestial  bodies 


Fig.    56.  —  Star    cluster   in    Hercules.      Messier    13.     Taken    at   the 
Yerkes  Observatory.     Scale,   1  mm.  =  9.22  sec.  of  arc 

which  have  cut  their  way  through  widely  expanded 
nebular  masses  (Fig.  54)  has  been  entertained  for  a  long 
time. 

The  smaller  and  more  slowly  moving  immigrants,  on 
the  other  hand,   are  stopped   by   the   particles  of  the 

173 


WORLDS   IN   THE  MAKING 

nebulae.  We  therefore  see  the  stars  more  sparsely  dis- 
tributed in  the  immediate  neighborhood  of  the  nebulae, 
while  in  the  nebulae  themselves  they  appear  more  densely 
crowded.  This  fact  had  struck  Herschel  in  his  observa- 
tions of  nebulae;  in  recent  days  it  has  been  investigated 
by  Courvoisier  and  M.  Wolf.  In  this  way  several  centres 
of  attraction  are  created  in  a  nebula;  they  condense  the 
gases  surrounding  the  nebula,  and  catch,  so  to  say,  any 
stray  meteorites  and  collect  them  especially  in  the  inner 
portions  of  the  nebula.  We  frequently  observe,  further, 
how  the  nebular  matter  appears  attenuated  at  a  certain 
distance  from  the  luminous  bright  stars  (compare  Figs. 
52  and  55).  Finally,  the  nebulae  change  into  star  clusters 
which  still  retain  the  characteristic  shapes  of  the  nebulae; 
of  these  the  spiral  is  the  most  usual,  while  we  also  meet 
with  conical  shapes,  originating  from  conical  nebulae,  and 
spherical  shapes  (compare  Figs.  56,  57,  and  58). 

This  is,  broadly,  the  type  of  evolution  through  which 
Herschel,  relying  upon  his  observations,  presumed  a 
nebula  to  pass.  He  was,  however,  under  the  impression 
that  the  nebulous  matter  would  directly  be  condensed 
into  star  clusters  without  the  aid  of  strange  celestial 
immigrants. 

It  has  been  known  since  the  most  ancient  times,  and  has 
been  confirmed  by  the  observations  of  Herschel  and  others 
in  a  most  convincing  manner,  that  the  stars  are  strongly 
concentrated  about  the  middle  line  of  the  Milky  Way. 
It  is  not  improbable  that  there  was  originally  a  nebula 
of  enormous  dimensions  in  the  plane  of  the  Milky  Way, 
produced  possibly  by  the  collision  of  two  such  giant 
suns  as  Arcturus.  This  gigantic  nebula  has  gathered  up 
the  smaller  migrating  celestial  bodies  which,  in  their  turn, 
have  condensed  upon  themselves  nebular  matter,  and 

have  thereby  become  incandescent,  if  they  were  not  so 

174 


ORIGIN    OF    NEBULA 

before.  The  rotational  movement  in  those  parts  which 
were  far  removed  from  the  centre  of  the  Milky  Way  may 
be  neglected.  At  a  later  period  collisions  succeeded  be- 
tween the  single  stars  which  had  been  gathered  up,  and 
it  is  for  this  reason  that  gaseous  nebulae,  as  well  as  new 
stars,  are  comparatively  frequent  phenomena  in  the  plane 
of  the  Milky  Way.  This  view  may  some  day  receive 


Fig.  57. — Star  cluster  in  Pegasus.     Messier  15.    Taken  at  the  Yerkes 
Observatory.     Scale,  1  mm. =6.4  sec.  of  arc 

confirmation,  when  we  succeed  in  proving  the  existence 
of  a  central  body  in  the  Milky  Way,  evidence  of  which 

175 


WORLDS    IN    THE   MAKING 


might  possibly  be  deduced  from  the  curvature  of  the 
orbits  of  the  sun  or  of  other  stars. 

As  regards  the  ring-shaped  nebula  in  the  Lyre  (Fig. 
50),  the  most  recent  measurements  made  by  Newkirk 

point  to  the  result  that  the 
star  visible  in  its  centre  is 
distant  from  us  about  thirty- 
two  light-years.  As  it  ap- 
pears probable  that  this  star 
really  forms  the  central  core 
of  the  nebula,  the  distance  of 
the  nebula  itself  must  be 
thirty- two  light-years.  From 
the  diameter  of  the  ring- 
shaped  nebula  which  Newkirk 
estimates  at  one  minute  of 
arc,  this  astronomer  has  cal- 
culated that  the  distance  of 
the  ring  from  its  central  bo;ly 
is  equal  to  about  three  hun- 
dred times  the  radius  of  the  earth's  orbit — that  is  to 
say,  the  ring  is  about  ten  times  as  far  from  its  sun  as 
Neptune  is  from  our  sun.  There  is  a  faint  luminescence 
within  this  ring.  The  nebular  matter  may  originally 
have  been  more  concentrated  at  this  spot  than  in  the 
outer  portions  of  the  ring  itself.  But  this  mass  was  prob- 
ably condensed  on  meteors  which  immigrated  from  out- 
side, and  when  these  meteors  coalesced  dark  planets  were 
produced  which  move  about  the  central  body,  and  which 
have  gathered  about  them  most  of  the  gases.  If  that 
central  body  were  as  heavy  as  our  sun,  the  matter  in 
the  ring  should  revolve  about  it  in  five  thousand  years. 
That  rotation  would  suffice  to  wipe  out  the  original  spiral 
shape,  enough  of  which  has  yet  been  left  to  permit  of  our 

176 


Fig.   58. — Cone-shaped    star 
cluster  in  Gemini. 


ORIGIN  OF  NEBULA 

distinctly  discerning  the  two  wings  of  the  spiral.  The 
central  body  of  this  ring-shaped  nebula  gives  a  continuous 
spectrum  of  bright  lines  which  is  particularly  developed 
on  the  violet  side.  The  star  would  therefore  appear  to 
be  much  younger  and  much  hotter  than  our  sun,  and  its 
radiation  pressure  would  therefore  be  much  more  intense. 
The  period  of  rotation  of  the  nebula  may,  for  this  reason, 
have  to  be  estimated  at  a  considerably  higher  figure. 

The  eminent  Dutch  astronomer  Kapteyn  has  deduced 
from  the  proper  motions  of  168  nebulae  that  their  average 
distance  from  the  earth  is  about  seven  hundred  light-years 
and  equal  to  that  of  stars  of  the  tenth  magnitude.  The 
old  idea,  that  the  nebulae  must  be  infinitely  farther  re- 
moved from  us  than  the  fainter  stars,  would  therefore  ap- 
pear to  be  erroneous.  According  to  the  measurements  of 
Professor  Bohlin,  the  nebula  in  Andromeda  may  indeed  be 
at  a  distance  of  not  more  than  forty  light-years. 

The  "new  stars"  form  a  group  among  the  peculiar 
celestial  bodies  which  on  account  of  their  variable  light 
intensity  have  been  designated  as  "variable  stars,"  and 
of  which  a  few  typical  cases  should  be  mentioned,  because 
a  great  scientific  interest  attaches  to  these  problems.  The 
star  Eta,  in  Argus,  may  be  said  to  illustrate  the  strange  fate 
that  a  star  has  to  pass  through  when  it  has  drifted  into  a 
nebula  filled  with  immigrated  celestial  bodies.  It  is  one 
of  the  most  peculiar  variable  stars.  The  star  shines 
through  one  of  the  largest  nebular  clouds  in  the  heavens. 
Whether  it  stands  in  any  physical  connection  with  its 
surroundings  cannot  be  stated  without  further  examina- 
tion. The  star  might,  for  instance,  be  at  a  considerable 
distance  in  front  of  the  nebula,  between  the  latter  and 
ourselves.  Its  frequent  change  in  light  intensity  sug- 
gests, however,  a  series  of  collisions,  which  do  not  ap- 
pear unnatural  to  us  when  we  suppose  that  the  star  is 

177 


WORLDS    IN    THE    MAKING 

within  a  nebula  into  which  many  celestial  bodies  have 
drifted. 

As  this  star  belongs  to  the  southern  hemisphere,  it  was 
not  observed  before  our  astronomers  commenced  to  visit 
that  hemisphere.  In  1677  it  was  classed  as  a  star  of  the 
fourth  magnitude;  ten  years  later  it  was  of  the  second 
magnitude;  the  same  in  1751.  In  1827  it  was  of  the 
first  magnitude,  and  it  was  found  to  be  variable — that 
is  to  say,  it  shone  with  variable  brightness.  Herschel  ob- 
served that  it  fluctuated  between  the  first  and  second 
magnitudes,  and  that  it  increased  in  brightness  after 

1837,  so  that  it  was  by  1838  of  magnitude  0.2.     After 
that  it  began  to  decrease  in  intensity  up  to  April,  1839, 
when  it  had  the  magnitude  1.1.     It  remained  for  four 
years  approximately  at  this  intensity;   then  it  increased 
rapidly  again  in   1843,   and  surpassed  all  stars  except 
Sirius  (magnitude— 1.7). l    Afterwards  its  intensity  slow- 
ly diminished  once  more,  so  that  it  remained  just  visible 
to  the  naked  eye  (sixth  magnitude) ;  by  1869  it  had  be- 
come invisible.     Since  then  it  has  been  fluctuating  be- 
tween the  sixth  and  seventh  magnitudes. 

The  last  changes  in  the  intensity  of  this  star  strongly 
recall  the  behavior  of  the  new  star  in  Perseus,  only  that 
the  latter  has  been  passing  through  its  phases  at  a  much 
more  rapid  rate.  It  appears  to  be  certain,  however,  that 
Eta,  in  Argus,  was  from  the  very  beginning  far  brighter 
than  Nova  Persei,  and  that  at  least  once  before  the  great 
collision  in  1843  (after  which  it  was  surrounded  by  ob- 
scuring clouds  of  increasing  opacity) — namely,  in  January, 

1838,  it  had  been  exposed  to  a  slight  collision  of  quickly 

1  This  figure,  -1.7,  signifies  that  the  brightness  of  Sirius  is  2.522-7  =  12 
times  greater  than  that  of  a  star  of  magnitude  1.  Next  to  Sirius 
comes  Canopus,  with  magnitude  —1.0,  being  6.3  times  brighter  than 
a  star  of  magnitude  1. 

178 


ORIGIN  OF  NEBULAE 

vanishing  effect.  This  lesser  collision  was  probably  of 
the  kind  which  Mayer  imagined  for  the  earth  and  sun. 
It  would  give  rise  to  heat  development  corresponding  to 
the  heat  expenditure  of  the  sun  in  about  a  hundred  years. 
As  it  had  been  observed  that  the  star  was  variable  in 
an  irregular  manner  before  that,  we  ma}^  perhaps,  pre- 
sume that  it  had  already  undergone  another  collision. 

According  to  the  observations  of  Borisiak,  a  student 
in  Kief,  the  new  star  in  Perseus  would  have  been,  on  the 
evening  of  February  21,  1901,  of  1.5  magnitude,  while 
a  few  hours  previously  it  had  been  of  magnitude  12,  and 
the  following  evening  of  magnitude  2.7;  afterwards  its 
intensity  increased  up  to  the  following  evening,  when  it 
outshone  all  the  other  stars  in  the  northern  sky.  If  this 
statement  is  not  based  on  erroneous  observations,  the 
new  star  must  have  been  in  collision  with  another  celestial 
body  two  days  before  its  great  collision,  perhaps  with  a 
small  planet  in  the  neighborhood  of  the  sun,  with  which 
it  later  collided.  That  would  account  for  its  temporary 
brilliancy. 

New  stars  are  by  no  means  so  rare  as  one  might  perhaps 
assume.  Almost  every  year  some  new  star  is  discovered. 
By  far  most  of  these  are  seen  in  the  neighborhood  of  the 
Milky  Way,  where  the  visible  stars  are  unusually  crowded, 
so  that  a  collision  which  would  become  visible  to  us  may 
easily  occur. 

For  similar  reasons  we  find  there  also  most  of  the 
gaseous  nebulae. 

Most  of  the  star  clusters  are  also  in  the  neighborhood 
of  the  Milky  Way.  This  is  in  consequence  of  the  facts 
just  alluded  to.  The  nebulae  which  are  produced  by 
collisions  between  two  suns  are  soon  crossed  by  migrat- 
ing celestial  bodies  such  as  meteorites  or  comets,  which 
there  occur  in  large  numbers;  by  the  condensing  action 

179 


WORLDS   IN   THE   MAKING 

of  these  intruders  they  are  then  transformed  into  star 
clusters.  In  parts  of  the  heavens  where  stars  are  rel- 
atively sparse  (as  at  a  great  distance  from  the  Milky 
Way),  most  of  the  nebulae  observed  exhibit  stellar  spectra. 
They  are  nothing  but  star  clusters,  so  far  removed  from 
us  that  the  separate  stars  can  no  longer  be  distinguished. 
That  single  stars  and  gaseous  nebulae  are  so  rarely  per- 
ceived in  these  regions  is,  no  doubt,  due  to  their  great 
distance. 

Among  the  variable  stars  we  find  quite  a  number  which 
display  considerable  irregularity  in  their  fluctuations  of 
brightness,  and  which  remind  us  of  the  new  stars.  To 
this  class  belongs  the  just-mentioned  star  Eta,  in  Argus. 
Another  example  (the  first  one  which  was  recognized  as 
"variable")  is  Mira  Ceti,  which  may  be  translated,  "The 
Wonderful  Star  in  the  Constellation  of  the  Whale."  This 
mysterious  body  was  discovered  by  the  Frisian  priest 
Fabricius,  on  August  12,  1596,  as  a  star  of  the  second 
magnitude.  The  priest,  an  experienced  astronomer,  had 
not  previously  noticed  this  star,  and  he  looked  for  it  in 
vain  in  October,  1597.  In  the  years  1638  and  1639  the 
variability  of  the  star  was  recognized,  and  it  was  soon 
ascertained  to  be  irregular.  The  period  has  a  length  of 
about  eleven  months,  but  it  fluctuates  irregularly  about 
this  figure  as  a  mean  value.  At  its  greatest  intensity 
the  star  ranks  with  those  of  the  first  or  second  order. 
Sometimes  it  is  weaker,  but  it  is  always  of  more  than  the 
fifth  magnitude.  Ten  weeks  after  a  maximum  the  star 
is  no  longer  visible,  and  its  brightness  may  diminish  to 
magnitude  9.5.  In  other  words,  its  intensity  varies  about 
in  the  ratio  of  1  : 1000  (or  possibly  more).  After  a  mini- 
mum the  brightness  once  more  increases,  the  star  be- 
comes visible  again — that  is  to  say,  it  attains  the  sixth 
magnitude  —  and  after  another  six  weeks  it  will  once 

180 


ORIGIN    OF   NEBULA 

more  be  at  its  maximum.     We  have  evidently  to  deal 
with  several  superposed  periods. 

The  spectrum  of  this  star  is  rather  peculiar.  It  be- 
longs to  the  red  stars  with  a  band  spectrum  which  is 
crossed  by  bright  hydrogen  lines.  The  star  is  receding 
from  us  with  a  velocity  of  not  less  than  63  km.  (39  miles) 
per  second.  The  bright  hydrogen  lines  which  correspond 
to  the  spectrum  of  the  nebula  may  sometimes  be  resolved 
into  three  components,  of  which  the  middle 'one  corre- 
sponds to  a  mean  velocity  of  60  km.,  and  the  two  others 
have  variable  receding  velocities  of  35  and  82  km. — that 
is  to  say,  velocities  of  25  or  22  km.  less  or  more  than  the 
mean  velocity.  Evidently  the  star  is  surrounded  by  three 
nebulae;  one  is  concentrated  about  its  centre;  the  two 
others  lie  on  a  ring  the  matter  of  which  has  been  con- 
centrated on  two  opposite  sides.  The  ring,  which  recalls 
the  ring  nebula  in  the  Lyre,  seems  to  move  about  the  star 
with  a  velocity  of  23.5  km.  per  second.  As  this  revolution 
is  accomplished  within  eleven — or,  more  correctly,  within 
twenty-two  months,  since  there  must  be  two  maxima 
and  two  minima  during  one  revolution — the  total  circum- 
ference of  the  ring  will  be  23.5  x  86,400  x  670—1361 
millions,  and  the  radius  of  its  orbit  217  million  km., 
which  is  1.45  times  greater  than  the  radius  of  the  earth's 
orbit.  Now  the  velocity  of  the  earth  in  its  orbit  is  29.5 
km.  (18.3  miles)  per  second.  A  planet  at  1.45  times  that 
distance  from  the  sun  would  have  the  (1.203  times  smaller) 
velocity  of  24.5  km.  per  second,  which  is  approximately 
that  of  the  hypothetical  ring  of  Mira  Ceti.  We  conclude, 
therefore,  that  the  mass  of  the  central  sun  in  Mira  Ceti 
will  nearly  equal  the  mass  of  our  sun.  The  calculation 
really  suggests  that  Mira  would  be  about  eight  per  cent, 
smaller;  but  the  difference  lies  within  the  range  of  the 
probable  error. 

181 


WORLDS    IN   THE   MAKING 

Chandler  has  directed  attention  to  a  striking  regularity 
in  these  stars.  The  longer  the  period  of  their  variation, 
the  redder  in  general  their  color.  This  is  easily  compre- 
hended. The  denser  the  original  atmosphere,  the  more 
widely  the  gases  will  have  extended  outward  from  the 
star,  and  the  more  dust  will  have  been  caught  or  secreted 
by  it.  We  have  seen  that  the  limb  of  the  sun  has  a 
reddish  light  because  of  the  quantities  of  dust  in  the  solar 
atmosphere.  The  effect  is  chiefly  to  be  ascribed  to  the 
absorption  of  the  blue  rays  by  the  dust;  but  it  may 
partly  be  explained  on  the  assumption  that  the  solar 
radiations  render  the  dust  incandescent,  though  its  tem- 
perature may  be  lower  than  that  of  the  photosphere,  be- 
cause the  dust  lies  outside  the  sun,  and  that  it  will  there- 
fore emit  a  relatively  reddish  light.  The  more  dust  there 
is  in  a  nebula,  the  redder  will  be  its  luminescence ;  and  as 
the  quantity  of  dust  increases  in  general  with  the  extension 
of  the  nebula,  that  star  which  is  surrounded  by  wider 
rings  of  nebulae  will  in  general  be  more  red ;  but  the  greater 
the  radius  of  the  ring,  the  longer  also  will  in  general  be 
its  period. 

The  so-called  red  stars  show,  in  addition  to  the'  bright 
hydrogen  lines,  banded  spectra  which  indicate  the  pres- 
ence of  chemical  compounds.  On  this  account  such  stars 
were  formerly  credited  with  a  lower  temperature.  But 
the  same  peculiarity  is  also  observed  in  sun-spots,  al- 
though the  latter,  on  account  of  their  position,  must  have 
a  higher  temperature  than  the  surrounding  photosphere. 
The  presence  of  bands  in  the  spectrum  certainly  suggests 
high  pressure,  however.  The  red  stars  are  evidently  sur- 
rounded by  a  very  extensive  atmosphere  of  gases,  in  the 
inner  portions  of  which  the  pressure  is  so  high  that  the 
atoms  enter  into  combination.  The  spectra  of  the  red 

stars  display,  on  the  whole,  a  striking  resemblance  to 

182 


ORIGIN    OF    NEBULA 

those  of  the  sun-spots.  The  violet  portion  of  the  spectrum 
is  weakened,  because  the  masses  of  dust  have  extinguished 
this  light.  Owing  to  the  large  masses  of  dust  which  lie  in 
our  line  of  sight,  the  spectrum  lines  are  :n  both  cases 
markedly  widened  and  sometimes  accompanied  by  bright 
lines. 

Another  class  of  stars,  distinguished  by  bright  lines, 
comprises  those  studied  by  Wolf  and  Rayet,  and  named 
after  them.  These  stars  are  characterized  by  a  hydrogen 
atmosphere  of  enormous  extension,  large  enough  in  some 
cases,  it  has  been  calculated,  to  fill  up  the  orbit  of  Nep- 
tune. These  stars  are  evidently  either  hotter  and  more 
strongly  radiating  than  the  red  stars,  or  there  is  not 
so  much  dust  in  their  neighborhood — the  dust  may  pos- 
sibly have  been  expelled  by  the  strong  radiating  press- 
ure. They  are,  therefore,  classed  with  the  yellow,  and 
not  with  the  red  stars.  Although  there  is  every  reason 
to  suppose  that  their  central  bodies  are  at  least  as  hot  as 
those  of  the  white  stars,  the  dust  is  yet  able  to  reduce  the 
color  to  yellow,  owing  to  the  vast  extensions  of  their 
atmospheres. 

The  unequal  periods  in  stars  like  Mira  may  be  ex- 
plained by  the  supposition  that  there  are  several  rings 
of  dust  moving  about  them,  as  in  the  case  of  the  planet 
Saturn,  In  the  case  of  the  inner  rings  which  have  a  short 
period,  there  has  probably  been  sufficient  time  during 
the  uncounted  number  of  revolutions  to  effect  a  uniform 
distribution  of  the  dust.  Hence  we  do  not  discern  any 
noteworthy  nuclei  in  them,  such  as  we  observe  in  •  the 
tails  of  comets;  the  dust  rings  only  help  to  impart  to 
the  star  a  uniform  reddish  hue.  In  the  outer  rings  the 
distribution  of  dust  will,  however,  not  be  uniform.  One 
of  the  rings  may  be  responsible  for  the  chief  proper 

period.     By   the    co-operation   of   other   less  important 
'3  183 


WORLDS    IN    THE    MAKING 

dust  rings,  the  maximum  or  minimum,  we  shall  easily 
understand,  may  be  displaced,  and  thus  the  time  interval 
between  the  maxima  and  minima  be  altered.  This  al- 
teration of  the  period  is  so  strong  for  some  stars  that  we 
have  not  yet  succeeded  in  establishing  any  simple  perio- 
dicity. The  best-known  star  of  this  type  is  the  bright- 
red  star  Betelgeuse  in  the  constellation  of  Orion.  The 
brightness  of  this  star  fluctuates  irregularly  between  the 
magnitudes  1.0  and  1.4. 

By  far  the  largest  number  of  variable  stars  belong  to 
the  type  of  Mira.  Others  resemble  the  variable  star 
Beta  in  the  constellation  of  the  Lyre,  and  thus  belong  to 
the  Lyre  type.  The  variability  of  the  spectra  of  a  great 
many  of  these  stars  indicates  that  they  must  be  moving 
about  a  dark  star  as  companion,  or  rather  that  they  both 
move  about  a  common  centre  of  gravity.  The  change 
in  the  light  intensity  is,  as  a  rule,  explained  by  the  sup- 
position that  the  bright  star  is  partially  obscured  at 
times  by  its  dark  companion.  Many  irregularities,  how- 
ever, in  their  periods  and  other  circumstances  prove  that 
this  explanation  is  not  sufficient.  The  assumption  of 
rings  of  dust  circulating  about  the  star  and  of  larger 
condensation  centres  affords  a  better  elucidation  of  the 
variability  of  these  stars.  They  are  grouped  with  the 
white  or  yellow  stars,  in  whose  surroundings  the  dust 
does  not  play  so  large  a  part  as  in  that  of  Mira  Ceti. 
The  period  of  their  variability  is,  as  a  rule,  very  short, 
moreover — generally  only  a  few  days  (the  shortest  known, 
only  four  hours) — while  the  period  of  the  Mira  stars 
amounts  to  at  least  sixty-five  days,  and  may  attain  two 
years.  There  may  be  still  longer  periods  so  far  not  in- 
vestigated. 

Nearly  related  to  the  Lyre  stars  are  the  Algol  stars, 
whose  variability  can  be  explained  bv  the  assumption 

184 


ORIGIN    OF    NEBULAE 

that  another  bright  or  dark  star  is  moving  within 
their  vicinity,  partially  cutting  off  their  light.  There 
is  no  dust  in  these  cases,  and  the  spectrum  charac- 
terizes these  stars  as  stars  of  the  first  class — that  is,  as 
white  stars — so  far  as  they  have  been  studied  up  to  the 
present. 

We  must  presume  for  all  the  variable  stars  that  the 
line  of  sight  from  the  observer  to  the  star  falls  in  the 
plane  of  their  dust  rings  or  of  their  companions.  If  this 
were  not  so,  they  would  appear  to  us  like  a  nebula  with 
a  central  condensation  nucleus,  or,  so  far  as  Algol  stars 
are  concerned,  like  the  so-called  spectroscopic  doubles 
whose  motion  about  each  other  is  recognized  from  the 
displacement  of  their  spectral  lines. 

The  evolution  of  stars  from  the  nebulous  state  has  been 
depicted  by  the  famous  chief  of  the  Lick  Observatory,  in 
California,  W.  W.  Campbell,  as  follows  (compare  the 
spectra  of  the  stars  of  the  2d,  3d,  and  4th  class,  Figs.  59 
and  60) : 


440  450  460  470  480 


Fig.  59. — Comparison  of  spectra  of  stars  of  classes  2,  3,  4.  After 
photographs  taken  at  the  Yerkes  Observatory.  Blue  portions 
of  spectrum.  Wave-lengths  in  millionths  of  a  millimetre 

185 


WORLDS    IN    THE    MAKING 

520         530        540      550      5f>0     570     580 


Fig.  60. — Comparison  of  spectra  of  stars  of  classes  2,  3,  4.  After 
photographs  taken  at  the  Yerkes  Observatory.  Green  and  yellow 
portions  of  spectrum.  Wave-lengths  in  millionths  of  a  millimetre 

"It  is  not  difficult  to  select  a  long  list  of  well-known 
stars  which  cannot  be  far  removed  from  nebular  condi- 
tions. These  are  the  stars  containing  both  the  Hug- 
gins  and  the  Pickering  series  of  bright  hydrogen  lines, 
the  bright  lines  of  helium,  and  a  few  others  not  yet  iden- 
tified. Gamma  Argus  and  Zeta  Puppis  are  of  this  class. 
Another  is  DM  +30.3639°,  which  is  actually  surrounded 
with  a  spherical  atmosphere  of  hydrogen  some  five  sec- 
onds of  arc  in  diameter.  A  little  further  removed  from 
the  nebular  state  are  the  stars  containing  both  bright 
and  dark  hydrogen  lines — caught,  so  to  speak,  in  the  act 
of  changing  from  bright-line  to  dark-line  stars.  Gamma 
Cassiopeia,  Pleione,  and  My  Centauri  are  examples. 
Closely  related  to  the  foregoing  are  the  helium  stars. 
Their  absorption  lines  include  the  Huggins  hydrogen  series 
complete,  a  score  or  more  of  the  conspicuous  helium  lines, 

186 


ORIGIN    OF   NEBULA 

frequently  a  few  of  the  Pickering  series,  and  usually  some 
inconspicuous  metallic  lines.  The  white  stars  in  Orion 
and  in  the  Pleiades  are  typical  of  this  age. 

"  The  assignment  of  the  foregoing  types  to  an  early  place 
in  stellar  life  was  first  made  upon  the  evidence  of  the 
spectroscope.  The  photographic  discovery  of  nebulous 
masses  in  the  regions  of  a  large  proportion  of  the  bright- 
line  and  helium  stars  affords  extremely  strong  confirma- 
tion of  their  youth.  Who  that  has  seen  the  nebulous 
background  of  Orion  (Fig.  51)  or  the  remnants  of  neb- 
ulosity in  which  the  individual  stars  of  the  Pleiades 
(Fig.  52)  are  immersed  can  doubt  that  the  stars  in  these 
groups  are  of  recent  formation? 

"With  the  lapse  of  time,  stellar  heat  radiates  into 
space,  and,  so  far  as  the  individual  star  is  concerned,  is 
lost.  On  the  other  hand,  the  force  of  gravity  on  the 
surface  strata  increases.  The  inevitable  contraction  is 
accompanied  by  increasing  average  temperature.  Changes 
in  the  spectrum  are  the  necessary  consequence.  The  sec- 
ond hydrogen  series  vanishes,  the  ordinary  hydrogen  ab- 
sorption is  intensified,  the  helium  lines  become  indistinct, 
and  calcium  and  iron  absorptions  begin  to  assert  them- 
selves. Vega  and  Sirius  are  conspicuous  examples  of  this 
period.  Increasing  age  gradually  robs  the  hydrogen  lines 
of  their  importance,  the  H  and  K  lines  broaden,  the  metal- 
lic lines  develop,  the  bluish-white  color  fades  in  the  direc- 
tion of  the  yellow,  and,  after  passing  through  types 
exemplified  by  many  well-known  stars,  the  solar  stage  is 
reached.  The  reversing  layer  in  solar  stars  represents 
but  four  or  five  hydrogen  lines  of  moderate  intensity ;  the 
calcium  lines  are  commandingly  permanent,  and  some 
twenty  thousand  metallic  lines  are  visible.  The  solar 
type  seems  to  be  near  the  summit  of  stellar  life.  The 
average  temperature  of  the  mass  must  be  nearly  a  max- 

187 


WORLDS  IN  THE  MAKING 

imum ;  for  the  low  density  indicates  a  constitution 
that  is  still  gaseous  [compare  Chapter  VII.]. 

"  Passing  time  brings  a  lowering  of  the  average  tempera- 
ture. The  color  passes  from  yellow  to  red,  in  consequence 
of  lower  radiation,  temperature,  and  increasing  general 
absorption  by  the  atmosphere.  The  hydrogen  lines  be- 
come indistinct,  metallic  absorption  remains  permanent, 
and  broad  absorption  bands  are  introduced.  In  one 
type  (Secchi's  Type  III.),  of  which  Alpha  Herculis  is  an 
example,  these  bands  are  of  unknown  origin.  In  another 
class  (Secchi's  Type  IV.),  illustrated  by  the  star  19  Pis- 
cium,  they  have  been  definitely  identified  as  of  carbon 
origin. 

"  There  is  scarcely  room  for  doubt  that  these  types  of 
stars  (Type  IV.)  are  approaching  the  last  stages  of  stellar 
development.  Surface  temperatures  have  been  lowered 
to  the  point  of  permitting  more  complex  chemical  com- 
binations than  those  in  the  sun. 

"  Secchi's  Type  III.  includes  the  several  numbered  long- 
period  variable  stars  of  the  Mira  Ceti  class,  whose  spectra 
at  maximum  brilliancy  show  several  bright  lines  of 
hydrogen  and  other  chemical  elements.1  It  is  significant 
that  the  dull-red  stars  are  all  very  faint;  there  are  none 
brighter  than  magnitude  5.5.  Their  effective  radiatory 
power  is  undoubtedly  very  low." 

The  state  of  evolution,  which  succeeds  that  character- 

1  This  circumstance  indicates  that  the  red  color  of  these  stars,  as 
we  have  already  remarked  with  regard  to  Mira  Ceti,  is  not  to  be 
traced  back  to  a  low  temperature,  but  rather  to  the  dust  surround!  ng 
them.  The  most  extraordinary  brightness  of  some  stars,  like  Arc- 
turus  and  Betelgeuse,  which  are  redder  than  the  sun,  and  whose 
spectra,  according  to  Hale,  resemble  those  of  the  sun-spots,  pre- 
suppose a  very  high  temperature.  The  characteristic  lines  of  their 
spectra  are  produced  by  the  relatively  cool  vapors  of  their  outer 
portions. 

188 


ORIGIN    OF   NEBULA 

ized  as  the  Secchi  Type  IV.,  may  be  elucidated  with  the 
aid  of  the  examples  of  Jupiter  and  the  earth,  with  which 
we  are  more  familiar.  These  planets  would  be  invisible 
if  they  were  not  shining  in  borrowed  light. 

Jupiter  has  not  advanced  so  far  as  the  earth.  The 
specific  gravity  of  Jupiter  is  somewhat  lower  than  that 
of  the  sun  (1.27  against  1.38),  and,  apart  from  the  clouds 
in  its  atmosphere,  this  planet  is  probably  altogether  in 
a  gaseous  condition,  while  the  earth,  with  its  mean 
density  of  5.52,  possesses  a  solid  cold  crust,  enclosing 
its  incandescent  interior.  This  state  of  the  earth  cor- 
responds to  the  last  stage  in  the  evolution  of  the  stars. 

Of  the  streams  of  gaseous  matter  which  are  ejected 
when  stars  collide  with  one  another,  the  metallic  vapors 
are  rapidly  condensed  by  cooling;  only  helium  and  hy- 
drogen will  remain  in  the  gaseous  condition  and  form 
nebular  masses  about  the  central  body.  These  nebula? 
yield  bright  lights.  Their  luminosity  is  clue  to  the  nega- 
tive particles  which  are  sent  to  them  by  the  radiation 
pressure  of  near  stars,  and  especially  by  the  central  bodies 
of  the  nebula. 

With  the  new  stars  which  have  so  far  been  observed, 
this  pressure  of  radiation  soon  diminishes,  and  the  nebu- 
lar light  likewise  decreases  in  such  cases.  In  other  in- 
stances, as  with  the  stars  characterized  by  bright  hydro- 
gen and  helium  lines,  the  radiation  of  the  central  body 
or  stars  in  their  vicinity  seems  to  be  maintained  at  full 
force  for  long  periods. 

The  nebulous  accumulations  of  helium  and  hydrogen 
will  gradually  escape  and  be  condensed  in  neaf-by  stars 
under  the  formation  of  "explosive"  compounds.  The 
tendency  to  enter  into  combination  seems  to  be  strongest 
in  the  case  of  helium;  it  disappears  first  from  the  stellar 

atmosphere.     That    helium    enters   into    compounds    at 

189 


WORLDS   IN   THE   MAKING 

high  temperatures  seems  to  follow  from  the  researches 
of  Ramsay,  Cooke,  and  Kohlschiitter. 

Hydrogen  will  afterwards  be  absorbed,  and  the  light 
of  the  central  body  will  then  show  the  predominating 
occurrence  of  the  vapors  of  calcium  and  of  other  metals 
in  its  atmosphere.  Simultaneously  with  these,  chemical 
compounds  will  be  noticed,  among  which  the  carbon  com- 
pounds will  play  an  important  part — in  the  outer  por- 
tions of  the  sun-spots,  in  the  stars  of  the  Secchi  Type  IV., 
as  well  as  in  the  gaseous  envelopes  of  the  cornets.1 

Finally  a  crust  will  form.     The  star  is  extinct. 

1  The  presence  of  carbon  bands  in  the  spectrum  need  not  be  taken 
as  a  mark  of  low  temperature.  Crew  and  Hale  have  observed  that 
these  bands  gradually  vanished  from  an  arc  spectrum  as  the  tem- 
perature was  lowered  by  decreasing  the  current  intensity. 


VII 

THE    NEBULAR    AND    THE    SOLAR    STATES 

WE  will  now  proceed  to  a  more  intimate  consideration 
of  the  chemical  and  physical  conditions  which  probably 
characterize  the  nebula}  in  distinction  from  the  suns. 
These  properties  differ  in  many  respects  essentially  from 
those  which  we  are  accustomed  to  associate  with  matter 
as  investigated  by  us,  which  may,  from  this  point  of  view, 
be  styled  relatively  concentrated. 

The  differences  must  be  fundamental.  For  the  motto 
of  Clausius,  which  comprises  the  sum  of  our  knowledge 
of  the  nature  of  heat,  cannot  apply  to  nebulse.  This 
motto  reads: 

"  The  energy  of  the  universe  is  constant.  The  entropy  of  the 
universe  tends  to  a  maximum." 

Everybody  understands  what  is  meant  by  energy.  We 
know  energy  in  many  forms.  The  most  important  are: 
energy  of  position  (a  heavy  body  has  larger  energy  by 
virtue  of  its  having  been  raised  to  a  certain  height  above 
the  surface  of  the  earth  than  when  it  is  lying  on  the 
surface);  energy  of  motion  (a  discharged  rifle-bullet  has 
an  energy  which  is  proportional  to  the  mass  of  the  bullet 
and  to  the  square  of  its  velocity);  energy  of  heat,  which 
is  regarded  as  the  energy  of  the  motion  of  the  smallest 
particles  of  a  body;  electrical  energy,  such  as  can,  for 
instance,  be  stored  in  an  accumulator  battery,  and  which, 

191 


WORLDS    IN   THE   MAKING 

like  all  other  modifications  of  energy,  may  be  converted 
into  energy  of  heat;  and  chemical  energy,  such  as  is 
displayed  by  a  mixture  of  eight  grammes  of  oxygen 
with  one  gramme  of  hydrogen,  which  can  be  trans- 
formed into  water  under  a  strong  evolution  of  heat. 
When  we  say  that  the  energy  of  a  system  to  which  energy 
is  not  imparted  from  outside  is  constant,  we  merely  mean 
that  the  different  forms  of  energy  of  the  separate  parts 
of  this  system  may  be  transformed  into  other  forms  of 
energy,  but  that  the  sum  total  of  all  the  energies  must 
always  remain  unchanged.  According  to  Clausius  this 
law  is  valid  throughout  the  infinite  space  of  the  universe. 

By  entropy  we  understand  the  quantity  of  heat  of  a 
body  divided  by  its  absolute  temperature.  If  a  quantity 
of  heat,  of  Q  calories,  of  a  body  at  a  temperature  of  100° 
(absolute  temperature,  373°)  passes  over  to  another  body 
of  0°  (absolute  temperature,  273°),  the  total  entropy  of 
the  two  will  have  been  decreased  by  yf^,  and  increased  by 
^TS.  As  the  latter  quantity  is  the  greater,  the  entropy 
of  the  whole  will  have  increased.  By  itself,  we  know, 
heat  always  passes,  either  by  radiation  or  by  conduction, 
from  bodies  of  higher  temperature  to  bodies  of  lower  tem- 
perature. That  evidently  implies  an  increase  in  entropy, 
and  it  is  in  agreement  with  the  law  of  Clausius  that 
entropy  tends  to  increase. 

The  most  simple  case  of  heat  equilibrium  is  that  in 
which  we  place  a  number  of  bodies  of  unequal  tempera- 
tures in  an  enclosure  which  neither  receives  heat  from 
outside  nor  communicates  heat  to  the  outside.  In  some 
way  or  other,  usually  by  conduction  or  radiation,  the 
heat  will  pass  from  the  warmer  to  the  colder  bodies, 
until  at  last  equilibrium  ensues  and  all  the  bodies  have 
the  same  temperature.  According  to  Clausius,  the  uni- 
verse tends  to  that  thermal  equilibrium.  If  it  be  ever 

192 


THE  NEBULAR  AND  THE  SOLAR  STATES 

attained,  all  sources  of  motion,  and  hence  of  light,  will 
have  been  exhausted.  The  so-called  "heat  -  death" 
(Warmetod)  will  have  come.  . 

If  Clausius  were  right,  however,  this  heat-death,  we 
may  object,  should  already  have  occurred  in  the  infinitely 
long  space  of  time  that  the  universe  has  been  in  existence. 
Or  we  might  argue  that  the  world  has  not  yet  been  in 
existence  sufficiently  long,  but  that,  anyhow,  it  had  a 
beginning.  That  would  contradict  the  first  part  of  the 
law  of  Clausius,  that  the  energy  of  the  universe  is  con- 
stant; for  in  that  case  all  the  energy  would  have  origi- 
nated in  the  moment  of  creation.  That  is  quite  incon- 
ceivable, and  we  must  hence  look  for  conditions  for  which 
the  entropy  law  of  Clausius  does  not  hold. 

The  famous  Scotch  physicist  Clerk-Maxwell  has  con- 
ceived of  such  a  case.  Imagine  a  vessel  which  is  divided 
by  a  partition  into  two  halves,  both  charged  with  a  gas 
of  perfectly  uniform  temperature.  Let  the  partition  be 
provided  with  a  number  of  small  holes  which  would  not 
allow  more  than  one  gas  molecule  to  pass  at  a  time.  In 
each  hole  Maxwell  places  a  small,  intelligent  being  (one 
of  his  "demons"),  which  directs  all  the  molecules  which 
enter  into  the  hole,  and  which  have  a  greater  velocity 
than  the  mean  velocity  of  all  the  molecules,  to  the  one 
side,  and  which  sends  to  the  other  side  all  the  molecules 
of  a  smaller  velocity  than  the  average.1  All  the  undesir- 
able molecules  the  demon  bars  by  means  of  a  little  flap. 
In  this  way  all  the  molecules  of  a  velocity  greater  than 
the  average  may  be  collected  in  the  one  compartment, 
and  all  the  molecules  of  a  lesser  velocity  in  the  other  com- 

1  The  kinetic  theory  of  gases  imagines  all  the  molecules  of  a  gas  to 
be  in  constant  motion.  The  internal  pressure  of  the  gas  depends 
upon  the  mean  velocity  of  the  particles;  but  some  particles  will  move 
at  a  greater,  and  some  at  a  smaller  velocity  than  the  average. — H.B. 

193 


WORLDS   IN  THE   MAKING 

partment.  In  other  words,  heat — for  heat  consists  in 
the  movements  of  molecules — will  pass  from  the  one 
constantly  cooling  side  to  the  other,  which  is  constantly 
raising  its  temperature,  and  which  must  therefore  become 
warmer  than  the  former. 

In  this  instance  heat  would  therefore  pass  from  a 
colder  to  a  warmer  body,  and  the  entropy  would  di- 
minish. 

Nature,  of  course,  does  not  know  any  such  intelligent 
beings.  Nevertheless,  similar  conditions  may  occur  in 
celestial  bodies  in  the  gaseous  state.  When  the  mole- 
cules of  gas  in  the  atmosphere  of  a  celestial  body  have 
a  sufficient  velocity — which  in  the  case  of  the  earth  would 
be  11  km.  (7  miles)  per  second — and  when  they  travel 
outward  into  the  most  extreme  strata,  they  may  pass 
from  the  range  of  attraction  out  into  infinite  space,  after 
the  manner  of  a  comet,  which,  if  endowed  with  sufficient 
velocity  when  near  the  sun,  must  escape  from  the  solar 
system.  According  to  Stoney,  it  is  in  this  way  that  the 
moon  has  lost  its  original  atmosphere.  This  loss  of  gas 
is  certainly  imperceptible  in  the  case  of  our  sun  and  of 
large  planets  like  the  earth.  But  it  may  play  an  impor- 
tant part  in  the  household  of  the  nebulae,  where  all  the 
radiation  from  the  hot  celestial  bodies  is  stored  up,  and 
where,  owing  to  the  enormous  distances,  the  restraining 
force  of  gravity  is  exceedingly  feeble.  Thus  the  nebulae 
will  lose  their  most  rapid  molecules  from  their  outer 
portions,  and  they  will  therefore  be  cooling  in  these  outer 
strata.  This  loss  of  heat  is  compensated  by  the  radiation 
from  the  stars.  If,  now,  there  were  only  nebulae  of  one 
kind  in  the  whole  universe,  those  escaped  molecules  would 
finally  land  on  some  other  nebula,  heat  equilibrium  would 
thus  be  established  between  the  different  nebulae,  and  the 

"  heat-death  "  be  realized.     But  we  have  already  remark- 
lot 


THE   NEBULAR   AND   THE    SOLAR   STATES 

ed  that  the  nebulae  enclose  many  immigrated  celestial 
bodies,  which  are  able  to  condense  the  gases  from  their 
neighborhood,  and  which  thereby  assume  a  higher  tem- 
perature. 

The  lost  molecules  of  gases  may  also  stray  into  the 
vast  atmosphere  of  these  growing  stars,  and  the  con- 
densation will  then  be  hastened  under  a  continuous  lower- 
ing of  the  entropy.  By  such  processes  the  clock-work 
of  the  universe  may  be  maintained  in  motion  without 
running  down. 

About  the  bodies  which  have  drifted  into  nebulae,  and 
about  the  remnants  of  new  stars  which  lie  inside  the 
nebulae,  the  gases  will  thus  collect  which  had  formerly 
been  scattered  through  the  outer  portions  of  the  nebula. 
These  gases  originate  from  the  explosive  compounds 
which  had  been  stored  in  the  interior  of  the  new  stars. 
Hydrogen  and  helium  are,  most  likely,  the  most  impor- 
tant of  these;  for  they  are  the  most  difficult  to  be  con- 
densed, and  can  exist  in  notable  quantities  at  extremely 
low  temperatures,  such  as  must  prevail  in  the  outermost 
portions  of  the  nebulae,  in  which  gases  of  other  substances 
would  be  liquefied.  Even  if  the  nebulae  had  an  absolute 
temperature  of  50°  (-223°  C.),  the  vapor  of  the  most 
volatile  of  all  the  metals,  mercury,  would  even  in  the 
saturated  state  be  present  in  such  a  small  quantity  that 
a  single  gramme  would  occupy  the  space  of  a  cube  whose 
side  would  correspond  to  about  two  thousand  light-years 
— that  is  to  say,  to  450  times  the  distance  of  the  earth 
from  the  nearest  fixed  star.  One  gramme  of  sodium,  like- 
wise a  very  volatile  metal,  and  of  a  comparatively  high 
importance  in  the  constitution  of  the  fixed  stars,  would 
fill  the  side  of  a  cube  that  would  be  a  thousand  million 
times  as  large.  Still  more  inconceivable  numbers  re- 
sult for  magnesium  and  iron,  which  are  very  frequent 

195 


WORLDS    IN    THE   MAKING 

constituents  of  fixed  stars,  and  which  are  less  volatile 
than  the  just-mentioned  metals.  We  thus  recognize  the 
strongly  selective  action  of  the  low  temperatures  upon 
all  the  substances  which  are  less  difficult  to  condense  than 
helium  and  hydrogen.  As  we  now  know  that  there  is 
another  substance  in  the  nebulae,  which  has  been  desig- 
nated nebulium,  and  which  is  characterized  by  two  spec- 
tral lines  not  found  in  any  terrestrial  substance,  we  must 
conclude  that  this  otherwise  unknown  element  nebulium 
must  be  almost  as  difficult  to  condense  as  hydrogen  and 
helium.  Its  boiling-point  will  probably  lie  below  50° 
absolute,  like  that  of  those  gases. 

That  hydrogen  and  helium,  together  with  nebulium, 
alone  seem  to  occur  in  the  vastly  extended  nebulae  is 
probably  to  be  ascribed  to  their  low  boiling-points.  We 
need  not  look  for  any  other  explanation.  The  supposi- 
tion of  Lockyer  that  all  the  other  elements  would  be  trans- 
formed into  hydrogen  and  helium  at  extreme  rarefaction 
is  quite  unsupported. 

In  somewhat  lower  strata  of  the  nebula,  where  its 
shape  resembles  a  disk,  other  not  easily  condensable 
substances,  such  as  nitrogen,  hydro-carbons  of  simple 
composition,  carbon  monoxide,  further,  at  deeper  levels, 
cyanogen  and  carbon  dioxide,  and,  near  the  centre,  sodium, 
magnesium,  and  even  iron  may  occur  in  the  gaseous  state. 
These  less  volatile  constituents  may  exist  as  dust  in  the 
outermost  strata.  This  dust  would  not  be  revealed  to  us 
by  the  spectroscope'.  In  the  strongly  developed  spiral 
nebulae,  however,  the  extreme  layers,  which  seem  to  hide 
the  central  body,  appear  to  be  so  attenuated  that  the 
dust  floating  in  them  is  not  able  to  obscure  the 
spectrum  of  the  metallic  gases.  The  spectrum  of  the 
nebula  then  resembles  a  star  spectrum,  because  the 
deepest  strata  contain  incandescent  layers  of  dust  clouds, 

196 


THE    NEBULAR   AND   THE   SOLAR   STATES 

whose   light   is   sifted  by  the   surrounding    masses    of 
gases. 

It  has  been  observed  that  the  lines  of  the  different 
elements  are  not  uniformly  distributed  in  the  nebula?. 
Thus  Campbell  observed,  for  instance,  when  investigating 
a  small  planetary  nebula  in  the  neighborhood  of  the  great 
Orion  nebula,  that  the  nebulium  had  not  the  same  dis- 
tribution as  the  hydrogen.  The  nebulium,  which  was  con- 
centrated in  the  centre  of  the  nebula,  probably  has  a 
higher  boiling-point  than  hydrogen,  therefore,  and  occurs 
in  noticeable  quantities  in  the  inner,  hotter  parts  of  the 
nebula.  Systematic  investigations  of  this  kind  may  help 
us  to  a  more  perfect  knowledge  of  the  temperature  rela- 
tions in  these  peculiar  celestial  objects. 

Ritter  and  Lane  have  made  some  interesting  calcula- 
tions on  the  equilibrium  in  a  gaseous  celestial  body  of 
so  low  a  density  that  the  law  of  gases  may  be  applied  to 
it.  That  is  only  permissive  for  gases  or  for  mixtures  of 
gases  whose  density  does  not  exceed  one-tenth  of  that 
of  water  or  one-fourteenth  of  the  actual  density  of  the 
sun.  The  pressure  in  the  central  portions  of  such  a  mass 
of  gas  would,  of  course,  be  greater  than  the  pressure  in 
the  outer  portions,  just  as  the  pressure  rises  as  we  pene- 
trate from  above  downward  into  our  terrestrial  atmos- 
phere. If  we  imagine  a  mass  of  the  air  of  our  atmosphere 
transferred  one  thousand  metres  higher  up,  its  volume  will 
increase  and  its  temperature  will  fall  by  9.8°  C.  (18°  F.). 
If  there  were  extremely  violent  vertical  convection  cur- 
rents in  the  air,  its .  temperature  would  diminish  in  this 
manner  with  increasing  altitude;  but  internal  radiation 
tends  to  equalize  these  temperature  differences.  The 
following  calculation  by  Schuster  concerning  the  con- 
ditions of  a  mass  of  gas  of  the  size  of  the  sun  is  based 
on  Ritter's  investigation.  It  has  been  made  under  the 

197 


\ 

WORLDS    IN    THE   MAKING 

hypothesis  that  the  thermal  properties  of  this  mass  of 
gas  are  influenced  only  by  the  movements  in  it,  and  not 
by  radiation.  The  calculation  is  applied  to  a  star  which 
has  the  same  mass  as  the  sun  (1.9x10"  grammes,  or 
324,000  times  the  mass  of  the  earth),  and  a  radius  of 
about  ten  times  that  of  the  sun  (10x690,000  km.),  whose 
mean  density  would  thus  be  1000  times  smaller  than  that 
of  the  sun,  or  0.0014  times  the  density  of  water  at  4°  C. 
In  the  following  table  the  first  column  gives  the  distance 
of  a  point  from  the  centre  of  the  star  as  a  fraction  of 
its  radius;  the  density  (second  column)  is  expressed  in 
the  usual  scale,  water  being  the  unit ;  pressures  are  stated 
in  thousands  of  atmospheres,  temperatures  in  thousands 
of  degrees  Centigrade.  The  temperature  will  vary  pro- 
portionately to  the  molecular  weight  of  the  gas  of  which 
the  star  consists;  the  temperatures,  in  the  fourth  column 
of  the  table,  concern  a  gas  of  molecular  weight  1 — that 
is  to  say,  hydrogen  gas  dissociated  into  atoms,  as  it 
will  be  undoubtedly  on  the  sun  and  on  the  star.  If  the 
star  should  consist  of  iron,  we  should  have  to  multiply 
these  latter  numbers  by  56,  the  molecular  weight  of  iron ; 
the  corresponding  figures  will  be  found  in  the  fifth  column. 


Temperature  in 

Distance  from 

Density 

Pressure  in  103 

1()30 

Cent. 

centre 

atmospheres 

Hydrogen 

Iron 

0 

0.00844 

852 

2460 

137.500 

0.1 

0.00817 

807 

2406 

131,600 

0.2 

0.00739 

683 

2251 

126,100 

0.3 

0.00623 

513 

2007 

112,400 

0.4 

0.00488 

342 

1707 

95,600 

0.5 

0.00354 

200 

1377 

77,100 

0.6 

0.00233 

100 

1043 

58,400 

0.7 

0.00136 

40 

728 

48,800 

0.8 

0.00065 

12 

445 

24,900 

0.9 

0.00020 

1.7 

202 

11,300 

1.0 

0.00000 

0 

0 

0 

Schuster's  calculation  was  really  made  for  the  sun— 
that  is  to  say,  for  a  celestial  body  whose  diameter  is  ten 

198 


THE    NEBULAR   AND    THE    SOLAR    STATES 

times  smaller,  and  whose  specific  gravity  is  therefore  a 
thousand  times  greater  than  the  above-assumed  values. 
According  to  the  laws  of  gravitation  and  of  gases,  the 
pressure  must  there  be  10,000  times  greater,  and  the 
temperature  ten  times  higher,  than  those  in  our  table. 
The  density  of  the  interior  portions  would,  however,  be- 
come far  too  large  to  admit  of  the  application  of  the 
gas  laws.  I  have  therefore  modified  the  calculations  so 
as  to  render  them  applicable  to  a  celestial  body  of  ten 
times  the  radius  of  the  sun  or  of  1080  times  the  radius 
of  the  earth ;  the  radius  would  then  represent  one-twenty- 
second  of  the  distance  from  the  centre  of  the  sun  to  the 
earth's  orbit,  and  the  respective  celestial  body  would  have 
very  small  dimensions  indeed  if  compared  to  a  nebula. 

The  extraordinarily  high  pressure  in  the  interior  por- 
tions of  the  celestial  body  is  striking;  this  is  due  to  the 
great  mass  and  to  the  small  distances.  In  the  centre 
of  the  sun  the  pressure  would  amount  to  8520  million 
atmospheres,  since  the  pressure  increases  inversely  as  the 
fourth  power  of  the  radius.  The  pressure  near  the  centre 
of  the  sun  is,  indeed,  almost  of  that  order.  If  the  sun 
were  to  expand  to  a  spherical  planetary  nebula  of  a 
thousand  times  its  actual  linear  dimensions  (when  it 
would  almost  fill  the  orbit  of  Jupiter),  the  specific  gravity 
at  its  centre  would  be  diminished  to  one-millionth  of  the 
above-mentioned  value — that  is  to  say,  matter  in  this 
nebula  would  not,  even  at  the  point  of  greatest  concen- 
tration, be  any  denser  than  in  the  highly  rarefied  vacuum 
tubes  which  we  can  prepare  at  ordinary  temperatures. 
The  pressure  would  likewise  be  greatly  diminished — 
namely,  to  about  six  millimetres  only,  near  the  centre  of 
the  gaseous  mass.  The  temperature,  however,  would  be 
rather  high  near  the  centre — namely,  24,600°  C.,  if  the 
nebula  should  consist  of  atomatic  hydrogen,  and  fifty-six 
14  199 


WORLDS   IN   THE   MAKING 

times  as  high  again  if  consisting  of  iron  gas.  Such  a 
nebula  would  restrain  gases  with  1.63  times  the  force 
which  the  earth  exerts.  Molecules  of  gases  moving  out- 
ward with  a  velocity  of  about  18  km.  (11  miles)  per  sec- 
ond would  forever  depart  from  this  atmosphere. 

The  estimation  of  the  temperature  in  such  masses  of 
gases  is  certainly  somewhat  unreliable.  We  have  to  pre- 
sume that  neither  radiation  nor  conduction  exert  any 
considerable  influence.  That  might  be  permitted  for 
conduction;  but  we  are  hardly  justified  in  neglecting 
radiation.  The  temperatures  within  the  interior  of  the 
nebula  will,  therefore,  be  lower  than  our  calculated  values. 
It  is,  however,  difficult  to  make  any  definite  allowance 
for  this  factor. 

If  the  mass  of  the  celestial  body  should  not  be  as 
presumed — for  instance,  twice  as  large — we  should  only 
have  to  alter  the  pressure  and  the  density  of  each  layer 
in  the  same  proportion,  and  thus  to  double  the  above 
values.  The  temperature  would  remain  unchanged. 
We  are  hence  in  a  position  to  picture  to  ourselves  the  state 
of  a  nebula  of  whatever  dimensions  and  mass. 

Lane  has  proved,  what  the  above  calculations  also  in- 
dicate, that  the  temperature  of  such  nebula  will  rise 
when  it  contracts  in  consequence  of  its  losing  heat.  If 
heat  were  introduced  from  outside,  the  nebula  would  ex- 
pand under  cooling.  A  nebula  of  this  kind  presumably 
loses  heat  and  gradually  raises  its  own  temperature  until 
it  has  changed  into  a  star,  which  will  at  first  have  an  at- 
mosphere of  helium  and  of  hydrogen  like  that  of  the 
youngest  stars  (with  white  light).  By-and-by,  under  a 
further  rise  of  temperature,  the  extremely  energetic 
chemical  compounds  will  be  formed  which  characterize  the 
interior  of  the  sun,  because  helium  and.  hydrogen — which 

were  liberated  when  the  nebula  was  re-formed  and  which 

200 


THE   NEBULAR   AND   THE   SOLAR   STATES 

dashed  out  into  space — will  diffuse  back  into  the  interior 
of  the  star,  where  they  will  be  bound  under  the  formation 
of  the  compounds  mentioned.  The  atmosphere  of  hy- 
drogen and  of  helium  will  disappear  (helium  first),  the 
star  will  contract  more  and  more,  and  the  pressure  and 
the  convection  currents  in  the  gases  will  become  enor- 
mous. There  will  be  a  strong  formation  of  clouds  in  the 
atmosphere  of  the  star,  which  will  gradually  become  en- 
dowed with  the  properties  which  characterize  our  sun. 
The  sun  behaves  very  differently  from  the  gaseous  nebulae 
for  which  the  calculations  of  Lane,  Ritter,  and  Schuster 
hold.  For  when  the  contraction  of  a  gas  shall  have  pro- 
ceeded to  a  certain  limit,  the  pressure  will  increase  in 
the  ratio  1 : 16,  while  the  volume  will  decrease  in  the  ratio 
8:1,  provided  there  be  no  change  in  the  temperature. 
When  the  gas  has  reached  this  point  and  is  still  further 
compressed,  the  temperature  will  remain  in  steady  equi- 
librium. At  still  higher  pressures,  however,  the  tempera- 
ture must  fall  if  equilibrium  is  to  be  maintained.  Accord- 
ing to  Amagat,  this  will  occur  at  17°  C.  (290°  absolute) 
in  gases  like  hydrogen  and  nitrogen,  which  at  this  tem- 
perature are  far  above  their  critical  points,  and  at  a  press- 
ure of  300  or  250  atmospheres.  When  the  temperature 
is  twice  as  high  on  the  absolute  scale,  or  at  307°  C., 
twice  the  pressure  will  be  required. 

We  can  now  calculate  when  our  nebula  will  pass 
through  this  critical  stage,  to  which  a  lowering  of  the 
temperature  must  succeed.  Accepting  the  above  figures, 
we  find  that  half  the  mass  of  the  nebula  will  fill  a  sphere 
of  a  radius  0.53  of  that  of  the  nebula.  If  the  mass  were 
everywhere  of  the  same  density,  half  of  it  would  fill  a 
sphere  of  0.84  of  this  radius.  When  will  the  interior 
mass  cross  the  boundary  of  the  above  stage,  while  the 
exterior  portions  still  remain  below  this  stage?  That 

201 


WORLDS    IN   THE   MAKING 

will  be  at  about  the  time  when  the  nebula  in  its  totality 
will  pass  through  its  maximum  temperature.  We  will 
now  base  our  calculations  on  the  temperatures  which 
apply  to  iron  in  the  gaseous  state;  for  in  the  interior  of 
the  nebula  the  mean  molecular  weight  will  at  least  be 
56  (that  of  iron).  We  shall  find  that  the  pressure  at 
the  distance  0.53  will  be  about  177,000  atmospheres,  and 
the  temperature  approximately  71  million  degrees — i.e., 
245,000  times  higher  than  the  absolute  temperature  in  the 
experiments  of  Amagat.  The  specified  stage  will  then 
be  reached  when  the  pressure  will  be  245,000  times  as 
large  as  250  atmospheres — viz.,  61  million  atmospheres. 
As,  now,  the  pressure  is  only  177,000  atmospheres,  our 
nebula  will  yet  be  far  removed  from  that  stage  at  which 
cooling  will  set  in.  We  can  easily  calculate  that  this 
will  take  place  when  the  nebula  has  contracted  to  a 
volume  about  three  times  that  of  our  sun.  The  assertion 
which  is  so  often  made  that  the  sun  might  possibly  attain 
higher  temperatures  in  the  future  is  unwarranted.  This 
celestial  body  has  long  since  passed  through  the  culminat- 
ing-point  of  its  thermal  evolution,  and  is  now  cooling. 
As  the  temperatures  which  Schuster  deduced  were  no 
doubt  much  too  high,  the  cooling  must,  indeed,  have 
set  in  already  in  an  earlier  stage.  But  stars  like  Sirius, 
whose  density  is  probably  not  more  than  one  per  cent, 
of  the  solar  density,  are  probably  still  in  a  rising-tempera- 
ture stage.  Their  condition  approximates  that  of  the 
mass  of  gas  of  our  example. 

The  planetary  nebulae  are  vastly  more  voluminous. 
The  immense  space  which  these  celestial  bodies  may  oc- 
cupy will  be  understood  from  the  fact  that  the  largest 
among  them,  No.  5  in  Herschel's  catalogue,  situated  near 
the  star  B  in  the  Great  Bear,  has  a  diameter  of  2.67 
seconds  of  arc.  If  it  were  as  near  to  us  as  our  nearest 

202 


THE    NEBULAR   AND   THE    SOLAR   STATES 

star  neighbor,  its  diameter  would  yet  be  more  than  three 
times  that  of  the  orbit  of  Neptune;  doubtless  it  is  many 
hundreds  of  times  larger.  This  consideration  furnishes 
us  with  an  idea  of  the  infinite  attenuation  in  such  struct- 
ures. In  their  very  densest  portions  the  density  cannot 
be  more  than  one-billionth  of  the  density  of  the  air. 
In  the  outer  portions  of  such  nebulae  the  temperature 
niust  also  be  exceedingly  low;  else  the  particles  of  the 
nebula  could  not  be  kept  together,  and  only  hydrogen 
and  helium  can  occur  in  them  in  the  gaseous  state. 

Yet  we  may  regard  the  density  and  temperature  of  such 
celestial  bodies  as  gigantic  by  comparison  with  those  of 
the  gases  in  the  spirals  of  the  nebulae.  There  never  is 
equilibrium  in  these  spirals,  and  it  is  only  because  the 
forces  in  action  are  so  extraordinarily  small  that  these 
structures  can  retain  their  shapes  for  long  periods  with- 
out noticeable  changes.  It  is,  probably,  chiefly  in  those 
parts  in  which  the  cosmical  dust  is  stopped  in  its  motion 
that  meteorites  and  comets  are  produced.  The  dust 
particles  wander  into  the  more  central  portions  of  the 
nebulae,  into  which  they  penetrate  deeply,  owing  to  their 
relatively  large  mass,  to  form  the  nuclei  for  the  growth 
of  planets  and  moons.  By  their  collisions  with  the  masses 
of  gases  which  they  encounter,  they  gradually  assume  a 
circular  movement  about  the  axis  of  rotation  of  the 
nebula.  In  this  rotation  they  condense  portions  of  the 
gases  on  their  surface,  and  hence  acquire  a  high  tem- 
perature— which  they  soon  lose  again,  however,  owing  to 
the  comparatively  rapid  radiation. 

So  far  as  we  know,  spiral  nebulae  are  characterized  by 
continuous  spectra.  The  splendor  of  the  stars  within 
them  completely  outshines  the  feeble  luminosity  of  the 
nebula.  The  stars  in  them  are  condensation  products 

and  undoubtedly  in  an  early  stage  of  their  existence; 

203 


WORLDS    IN   THE   MAKING 

they  may  therefore  be  likened  to  the  white  stars,  like 
the  new  star  in  Perseus  and  the  central  star  in  the 
ring  nebula  of  the  Lyre.  Nevertheless,  it  has  been  as- 
certained that  the  spectrum  of  the  Andromeda  nebula  has 
about  the  same  length  as  that  of  the  yellow  stars.  That 
may  be  due  to  the  fact  that  the  light  of  the  stars  in  this 
nebula,  which  we  only  seem  to  see  from  the  side,  is  partly 
extinguished  by  dust  particles  in  its  outer  portion,  as  was 
the  case  with  the  light  of  the  new  star  in  Perseus  during 
the  period  of  its  variability. 

Our  considerations  lead  to  the  conclusion  that  there 
is  rotating  about  the  central  body  of  the  nebula  an 
immense  mass  of  gas,  and  that  outside  this  mass  there 
are  other  centres  of  condensation  moving  about  the  cen- 
tral body  together  with  the  masses  of  gas  concentrated 
about  them.  Owing  to  the  friction  between  the  immi- 
grated masses  and  the  original  mass  of  gas  which  cir- 
culated in  the  equatorial  plane  of  the  central  body,  all 
these  masses  will  keep  near  the  equatorial  plane,  which 
will  therefore  deviate  little  from  the  ecliptic.  We  thus 
obtain  a  proper  planetary  system,  in  which  the  planets 
are  surrounded  by  colossal  spheres  of  gas  like  the  stars 
in  the  Pleiades  (Fig.  52).  If,  now,  the  planets  have  very 
small  mass  by  comparison  with  the  central  body — as  in 
our  solar  system — they  will  be  cooled  at  an  infinitely 
faster  rate  than  the  sun.  The  gaseous  masses  will  soon 
shrink,  and  the  periods  of  rotation  will  be  shortened;  but 
for  those  planets,  at  least,  which  are  situated  near  the 
centre,  these  periods  will  originally  differ  little  from  the 
rotation  of  the  central  body.  The  dimensions  of  the 
central  body  will  always  be  very  large,  and  the  planets 
circulating  about  it  will  produce  very  strong  tidal  effects 
in  its  mass.  Its  period  of  rotation  will  be  shortened, 

while  the  orbital  rotation  of  the  planets  will  tend  to  be- 

204 


THE  NEBULAR  AND  THE  SOLAR  STATES 

come  lengthened.  Thus  the  equilibrium  is  disturbed;  it 
is  re-established  again,  because  the  planet  is,  so  to  say, 
lifted  away  from  the  sun,  as  G.  H.  Darwin  has  so  in- 
geniously shown  with  regard  to  the  moon  and  the  earth. 
Similar  relations  will  prevail  in  the  neighborhood  of 
those  planets  which  will  thus  become  provided  with  moons. 
Hence  we  understand  the  peculiar  fact  that  all  the  planets 
move  almost  in  the  same  plane,  the  so-called  ecliptic, 
and  in  approximately  circular  orbits;  that  they  all  move 
in  the  same  direction,  and  that  they  have  the  same 
direction  of  rotation  in  common  with  their  moons  and 
with  the  central  body,  the  sun.  It  is  only  the  outermost 
planets,  like  Uranus  and  Neptune,  in  whose  cases  the  tidal 
effects  were  not  of  much  consequence,  that  form  excep- 
tions to  this  rule. 

In  explanation  of  these  phenomena  various  philosophers 
and  astronomers  have  advanced  a  theory  which  is  known 
as  the  Kant-Laplace  theory,  after  its  most  eminent  ad- 
vocates. Suggestions  pointing  in  the  same  direction  we 
find  in  Swedenborg  (1734).  Swedenborg  assumed  that 
our  planetary  system  had  been  evolved  under  the  forma- 
tion of  vortices  from  a  kind  of  "chaos  solare,"  which  had 
acquired  a  more  and  more  energetic  circulating  motion 
about  the  sun  under  the  influence  of  internal  forces, 
possibly  akin  to  magnetic  forces.  Finally  a  ring  had  been 
thrown  off  from  the  equator,  and  had  separated  into 
fragments,  out  of  which  the  planets  had  been  formed. 

Buffon  introduced  gravitation  as  the  conservational 
principle.  In  an  ingenious  essay,  "Formation  des  Pla- 
netes"  (1745),  he  suggests  that  the  planets  may  have 
been  formed  from  a  "stream"  of  matter  which  was 
ejected  by  the  sun  when  a  comet  rushed  into  it. 

Kant  started  from  an  original  chaos  of  stationary  dust, 

which  under  the  influence  of  gravitation  arranged  itself 

205 


WORLDS   IN   THE   MAKING 

as  a  central  body,  with  rings  of  dust  turning  around  it; 
the  rings,  later  on,  formed  themselves  into  planets.  The 
laws  of  mechanics  teach,  however,  that  no  rotation  can 
be  set  up  in  a  central  body,  which  is  originally  station- 
ary, by  the  influence  of  a  central  force  like  gravitation. 
Laplace,  therefore,  assumed  with  Swedenborg  that  the 
primeval  nebula  from  which  our  solar  system  was  evolved 
had  been  rotating  about  the  central  axis.  According  to 
Laplace,  rings  like  those  of  Saturn  would  split  off,  as 
such  a  system  contracted,  and  planets  and  their  moons 
and  rings  would  afterwards  be  formed  out  of  those  rings. 
It  is  generally  believed  at  present,  however,  that  only 
meteorites  and  small  planets,  but  not  the  larger  planets, 
could  have  originated  in  this  way.  We  have,  indeed, 
such  rings  of  dust  rotating  about  Saturn,  the  innermost 
more  rapidly,  the  outer  rings  more  slowly,  just  as  they 
would  if  they  were  crowds  of  little  moons. 

Many  further  objections  have  later  been  raised  against 
the  hypothesis  of  Laplace,  first  by  Babinet,  later  especially 
by  Moulton  and  Chamberlin.  In  its  original  shape  this 
hypothesis  would  certainly  not  appear  to  be  tenable. 
I  have  therefore  replaced  it  by  the  evolution  thesis  out- 
lined above.  It  is  rather  striking  that  the  moons  of  the 
outermost  planets,  Neptune  and  Uranus,  do  not  move 
in  the  plane  of  the  ecliptic,  and  that  their  moons  further 
describe  a  "retrograde"  movement — that  is  to  say,  they 
move  in  the  direction  opposite  to  that  conforming  to  the 
theory  of  Laplace.  The  same  seems  to  hold  for  the  moon 
of  Saturn,  which  was  discovered  in  1898  by  Pickering.  All 
these  facts  were,  of  course,  unknown  to  Laplace  in  1776; 
and  if  he  had  known  them  he  would  scarcely  have  ad- 
vanced his  thesis  in  the  garb  in  which  he  offered  it.  The 
explanation  of  these  facts  does  not  cause  any  difficulty. 

We  may  assume  that  the  matter  in  the  outer  portions  of 

206 


THE   NEBULAR   AND   THE    SOLAR   STATES 

the  primeval  nebula  was  so  strongly  attenuated  that  the 
immigrating  planet  did  not  attain  a  sufficient  volume  to 
have  the  large  common  rotation  in  the  equatorial  plane 
of  the  sun  impressed  upon  it  by  the  tidal  effects.  Charged 
only  with  the  small  mass  of  matter  which  they  met  on 
their  road,  the  planet  and  its  moon,  on  the  contrary,  re- 
mained victorious  in  the  limited  districts  in  which  they 
were  rotating.  Only  the  slow  orbital  movement  about 
the  central  body  was  influenced,  and  that  adapted  itself 
to  the  common  direction  and  the  circular  orbit.  It  is 
not  inconceivable  that  there  may  be,  farther  out  in  space, 
planets  of  our  solar  system,  unknown  to  us,  moving  in 
irregular  paths  like  the  comets.  The  comets,  Laplace 
assumed,  probably  immigrated  at  a  later  period  into  our 
solar  system  when  the  condensation  had  already  ad- 
vanced so  far  that  the  chief  mass  of  the  nebular  matter 
had  disappeared  from  interplanetary  space. 

Chamberlin  and  Moulton  have  attempted  to  show  that 
the  difficulties  of  the  hypothesis  of  Laplace  may  be 
obviated  by  the  assumption  that  the  solar  system  has 
evolved  from  a  spiral  nebula,  into  which  strange  bodies 
intruded  which  condensed  the  nebular  mass  of  their  sur- 
roundings upon  themselves.  We  have  pointed  out  ex- 
amples of  how  the  nebula  seems  to  vanish  in  the  vicinity 
of  the  stars,  which  would  correspond  to  growing  planets, 
located  in  nebulae. 

In  concluding  this  consideration,  we  may  draw  a  com- 
parison between  the  views  which  were  still  entertained 
a  short  time  ago  and  the  views  and  prospects  which  the 
discoveries  of  moderji  days  open  to  our  eyes. 

Up  to  the  beginning  of  this  century  the  gravitation  of 
Newton  seemed  to  rule  supreme  over  the  motions  and 
over  the  development  of  the  material  universe.  By  virtue 
of  this  gravitation  the  celestial  bodies  should  tend  to 

207 


WORLDS    IN   THE   MAKING 

draw  together,  to  unite  in  ever-growing  masses.  In  the 
infinite  space  of  past  time  the  evolution  should  have 
proceeded  so  far  that  some  large  suns,  bright  or  extinct, 
could  alone  persist.  All  life  would  be  impossible  under 
such  conditions. 

And  yet  we  discern  in  the  neighborhood  of  the  sun 
quite  a  number  of  dark  bodies,  our  planets,  and  we  may 
surmise  that  similar  dark  companions  or  satellites  exist 
in  the  vicinity  of  other  suns  and  stars;  for  we  could 
not  understand  the  peculiar  to-and-fro  motions  of  those 
stars  on  any  other  view.  We  further  observe  that  quite 
a  number  of  small  celestial  bodies  rush  through  space  in 
the  shapes  of  meteorites  or  shooting-stars  which  must 
have  come  to  us  from  the  most  remote  portions  of  the 
universe. 

The  explanation  of  these  apparent  deviations  from 
what  we  may  regard  as  a  necessary  consequence  of  the 
exclusive  action  of  gravity  will  be  found  under  two 
heads — in  the  action  of  the  mechanical  radiation  press- 
ure of  light,  and  in  the  collisions  between  celestial 
bodies.  The  latter  produce  enormous  vortices  of  gases 
about  nebular  structures  in  the  gaseous  condition;  the 
radiation  pressure  carries  cosmical  dust  into  the  vortices, 
and  the  dust  collects  into  meteorites  and  comets  and 
forms,  together  with  the  condensation  products  of  the 
gaseous  envelope,  the  planets  and  the  moons  accompany- 
ing them. 

The  scattering  influence  of  the  radiation  pressure  there- 
fore balances  the  tendency  of  gravitation  to  concentrate 
matter.  The  vortices  of  gases  in  the  nebula  only  servo 
to  fix  the  position  of  the  dust,  which  is  ejected  from  the 
suns  through  the  action  of  the  radiation  pressure. 

The  masses  of  gas  within  the  nebulae  form  the  most  im- 
portant centres  of  concentration  of  the  dust  which  is  eject- 

208 


THE    NEBULAR   AND   THE   SOLAR   STATES 

ed  from  the  sun  and  stars.  If  the  world  were  limited,  as 
people  used  to  fancy — that  is  to  say,  if  the  stars  were 
crowded  together  in  a  huge  heap,  and  only  infinite, 
empty  space  outside  of  this  heap,  the  dust  particles  ejected 
from  the  suns  during  past  ages  by  the  action  of  the  radiat- 
ing pressure  would  have  been  lost  in  infinite  space,  just 
as  we  imagined  that  the  radiated  energy  of  the  sun  was 
lost. 

If  that  were  so,  the  development  of  the  universe  would 
long  since  have  come  to  an  end,  to  an  annihilation  of  all 
matter  and  of  all  energy.  Herbert  Spencer,  among 
others,  has  explained  how  thoroughly  unsatisfactory  this 
view  is.  There  must  be  cycles  in  the  evolution  of  the 
universe,  he  has  emphasized.  That  is  manifestly  in- 
dispensable if  the  system  is  to  last.  In  the  more  rare- 
fied, gaseous,  cold  portions  of  the  nebulse  we  find  that 
part  of  the  machinery  of  the  universe  which  checks  the 
waste  of  matter  and,  still  more,  the  waste  of  force  from 
the  suns.  The  immigrating  dust  particles  have  absorbed 
the  radiation  of  the  sun  and  impart  their  heat  to  the 
separate  particles  of  the  gases  with  which  they  collide. 
The  total  mass  of  gas  expands,  owing  to  this  absorption 
of  heat,  and  cools  in  consequence.  The  most  energetic 
molecules  travel  away,  and  are  replaced  by  new  particles 
coming  from  the  inner  portions  of  the  nebulse,  which  are 
in  their  turn  cooled  by  expansion.  Thus  every  ray 
emitted  by  a  sun  is  absorbed,  and  its  energy  is  transferred, 
through  the  gaseous  particles  of  the  nebulse,  to  suns  that 
are  being  formed  and  which  are  in  the  neighborhood  of 
the  nebula  or  in  its  interior  portions.  The  heat  is  hence 
concentrated  about  centres  of  attraction  that  have 
drifted  into  the  nebula  or  about  the  remnants  of  the 
celestial  bodies  which  once  collided  there.  Thanks  to  the 
low  temperature  of  the  nebula,  the  matter  can  again  ac- 

209 


WORLDS    IN   THE   MAKING 

cumulate,  while  the  radiation  pressure,  as  Poynting  has 
shown,  will  suffice  to  keep  bodies  apart  if  their  tempera- 
ture is  15°  C.,  their  diameter  3.4  cm.,  and  their  specific 
gravity  as  large  as  that  of  the  earth,  5.5.  At  the  distance 
of  the  orbit  of  Neptune,  where  the  temperature  is  about 
50°  absolute  and  approximates,  therefore,  that  of  a 
nebula,  this  limit  of  size  is  reduced  to  nearly  one  milli- 
metre. It  has  already  been  suggested  (compare  page 
153)  that  capillary  forces,  which  would  prevail  under  the 
co-operation  of  the  gases  condensed  upon  the  dust  grains, 
rather  than  gravity,  play  a  chief  part  in  the  accumu- 
lation and  coalescence  of  the  small  particles.  In  the 
same  manner  as  matter  is  concentrated  about  centres  of 
attraction  energy  may  be  accumulated  there  in  contra- 
diction to  the  law  of  the  constant  increase  of  entropy. 

During  this  conservation al  activity  the  layers  of  gas 
are  rapidly  rarefied,  to  be  replaced  by  new  masses 
from  the  inner  parts  of  the  nebula,  until  this  centre  is 
depleted,  and  the  nebula  has  been  converted  into  a 
star  cluster  or  a  planetary  system  which  circulates  about 
one  or  several  suns.  When  the  suns  collide  once  more 
new  nebulae  are  created. 

The  explosive  substances,  consisting  probably  of  hy- 
drogen and  helium  (and  possibly  of  nebulium),  in  com- 
bination with  carbon  and  metals,  play  a  chief  part  in 
the  evolution  from  the  nebular  to  the  stellar  state,  and 
in  the  formation  of  new  nebulae  after  collisions  between 
two  dark  or  bright  celestial  bodies.  The  chief  laws  of 
thermodynamics  lead  to  the  assumption  that  these  ex- 
plosive substances  are  formed  during  the  evolution 
of  the  suns  and  are  destroyed  during  their  collisions. 
The  enormous  stores  of  energy  concentrated  in  these 
bodies  perform,  in  a  certain  sense,  the  duty  of  powerfully 
acting  fly  -  wheels  interposed  in  the  machinery  of  the 

210 


THE    NEBULAR   AND   THE   SOLAR    STATES 

universe  in  order  to  regulate  its  movements  and  to  make 
certain  that  the  cyclic  transition  from  the  nebular  to 
the  star  stage,  and  vice  versa,  will  occur  in  a  regular 
rhythm  during  the  immeasurable  epochs  which  we  must 
concede  for  the  evolution  of  the  universe. 

By  virtue  of  this  compensating  co-operation  of  gravity 
and  of  the  radiation  pressure  of  light,  as  well  as  of  tem- 
perature equalization  and  heat  concentration,  the  evolu- 
tion of  the  world  can  continue  in  an  eternal  cycle,  in 
which  there  is  neither  beginning  nor  end,  and  in  which 
life  may  exist  and  continue  forever  and  undiminished. 


VIII 

THE  SPREADING  OF  LIFE  THROUGH   THE   UNIVERSE 

WE  have  just  recognized  the  probability  of  the  assump- 
tion that  solar  systems  have  been  evolved  from  nebulae, 
and  that  nebulae  are  produced  by  the  collision  of  suns. 
We  likewise  consider  it  probable  that  there  circulate 
about  the  newly  formed  suns  smaller  celestial  bodies 
which  cool  more  rapidly  than  the  central  sun.  When 
these  satellites  have  provided  themselves  with  a  solid 
crust,  which  will  partly  be  covered  by  water,  they  may, 
under  favorable  conditions,  harbor  organic  life,  as  the 
earth  and  probably  also  Venus  and  Mars  do.  The  satel- 
lites would  thereby  gain  a  greater  interest  for  us  than 
if  we  had  to  imagine  them  as  consisting  entirely  of  life- 
less matter. 

The  question  naturally  arises  whether  we  may  believe 
that  life  can  really  originate  on  a  celestial  body  as  soon  as 
circumstances  are  favorable  for  its  evolution  and  prop- 
agation. This  question  will  occupy  us  in  this  last 
chapter. 

Men  have  been  pondering  over  these  problems  since  the 
remotest  ages.  All  living  beings,  past  ages  recognized, 
must  have  been  generated  and  they  had  to  die  after  a 
certain  shorter  or  longer  life.  Somewhat  later,  and  yet 
still  in  a  very  early  epoch,  experience  must  have  taught 
men  that  organisms  of  one  kind  can  only  generate  other 

organisms  of  the  same  kind;    that  the  species  are  in- 

212 


LIFE   THROUGH   THE    UNIVERSE 

variable,  as  we  now  express  it.  The  idea  was  that  all 
species  originally  came  from  the  hands  of  the  Creator 
endowed  with  their  present  qualities.  This  view  may 
still  be  said  to  represent  the  general  or  "orthodox"  doc- 
trine. 

This  view  has  also  been  called  the  Linnsean  thesis,  be- 
cause Linne,  in  the  fifth  edition  of  his  Genera  Plantarum, 
adheres  to  it  strictly:  "Species  tot  sunt,  quot  diversas 
formas  ab  initio  produxit  Infinitum  Ens,  quae  deinde 
formae  secundum  generationis  inditas  leges  produxere 
plures,  at  sibi  semper  similes,  ut  species  nunc  nobis  non 
sint  plures  quam  fuerunt  ab  initio."  Which  we  may 
render:  "There  are  as  many  different  kind  of  species  as 
the  Infinite  Being  has  created  different  forms  in  the 
beginning.  These  forms  have  later  engendered  other 
beings  according  to  the  laws  of  inheritance,  always  re- 
sembling them,  so  that  we  have  at  the  present  time  not 
any  more  species  than  there  were  from  the  beginning." 
Time  was  ripe,  however,  even  then  for  a  less  rigid  con- 
ception of  nature,  more  in  accordance  with  our  present 
views.  The  first  foundations  of  the  theory  of  evo- 
lution in  the  biological  sciences  were  laid  by  Lamarck 
(in  1794),  Treviranus  (in  1809),  Goethe  and  Oken  (in  1820). 
But  a  reaction  set  in.  Cuvier  and  his  authority  forced 
public  opinion  back  to  the  ancient  stand-point.  In  his 
view  the  now  extinct  species  of  past  geological  epochs 
had  been  destroyed  by  natural  revolutions,  and  new 
species  had  again  been  generated  by  a  new  act  of  the 
Creator. 

Within  the  last  few  decades,  however,  the  general  be- 
lief has  rapidly  been  revolutionized,  and  the  theory  of 
evolution,  especially  since  the  immortal  Charles  Darwin 
came  forth  with  his  epoch-making  researches,  now  meets 
with  universal  acceptance. 

213 


WORLDS    IN   THE   MAKING 

According  to  this  theory  the  species  adapt  themselves 
in  the  course  of  time  to  their  surroundings,  and  the 
changes  may  become  so  great  that  a  new  species  may  be 
considered  to  have  originated  from  an  old  species.  The 
researches  of  De  Vries  have,  within  quite  recent  times, 
further  accentuated  this  view,  so  that  we  now  concede 
cases  to  be  extant  where  new  species  spring  forth  from 
old  ones  under  our  very  eyes.  This  thesis  has  become 
known  as  the  theory  of  mutation. 

At  the  present  time  we  accordingly  imagine  that  living 
organisms,  such  as  we  see  around  us,  have  all  descended 
from  older  organisms,  rather  unlike  them,  of  which  we 
still  find  traces  and  remnants  in  the  geological  strata 
which  have  been  deposited  during  past  ages.  From  this 
stand-point  all  living  organisms  might  possibly  have 
originated  from  one  single,  most  simple  organism.  How 
that  was  generated  still  remains  to  be  explained. 

The  common  view,  to  which  the  ancients  inclined, 
is  that  the  lower  organisms  need  not  necessarily  have 
originated  from  seeds.  It  was  noticed  that  some  low- 
type  organisms,  larvae,  etc.,  took  rise  in  putrid  meat; 
Vergil  describes  this  in  his  Georgicas.  It  was  not 
until  the  seventeenth  century  that  this  belief  was  dis- 
proved by  many  experiments,  among  others  by  those 
of  Swammerdam  and  Leuwenhoek.  The  thesis  of  the 
so-called  "Generatio  spontanea"  once  more  blossomed 
into  new  life  upon  the  discovery  of  the  so-called  in- 
fusoria, the  small  animal  organisms  which  seem  to 
arise  spontaneously  in  infusions  and  concoctions.  Spal- 
lanzani,  however,  demonstrated  in  1777  that  when  the 
infusions,  and  the  vessel  containing  them,  as  well  as  the 
air  above  them,  were  heated  to  a  sufficiently  high  tem- 
perature to  kill  all  the  germs  present,  the  infusions  would 
remain  sterile,  and  no  living  organisms  could  develop 

214 


LIFE   THROUGH   THE    UNIVERSE 

in  them.  To  this  fact  we  owe  our  ordinary  methods  of 
making  preserves.  It  is  true  that  objections  were  raised 
against  this  demonstration.  The  air,  it  was  objected,  is 
so  changed  by  heating  that  subsequent  development  of 
minute  organisms  is  rendered  impossible.  But  this  last 
objection  was  refuted  by  the  chemists  Chevreul  and 
Pasteur,  as  well  as  by  the  physicist  Tyndall  in  the  sixties 
and  seventies  of  the  past  century.  These  scientists  dem- 
onstrated that  no  organisms  are  produced  in  air  which 
is  freed  from  the  smallest  germs  by  some  other  means  than 
heating — i.e.,  by  nitration  through  cotton-wool.  The 
researches  of  Pasteur,  in  particular,  and  the  methods  of 
sterilization  which  are  based  upon  them  and  which  are 
applied  every  day  in  bacteriological  laboratories,  have 
more  and  more  forced  the  conviction  upon  us  that  a  germ 
is  indispensable  for  the  origination  of  life. 

And  yet  eminent  scientists  take  up  the  pen  again  and 
again  in  order  to  demonstrate  the  possibility  of  the 
"Generatio  spontanea."  In  this  they  do  not  rely  upon 
the  safe  methods  of  natural  science,  but  they  proceed  on 
philosophical  lines  of  argument.  Life,  they  suggest, 
must  once  have  had  a  beginning,  and  we  are  hence  forced 
to  believe  that  spontaneous  generation,  even  if  not  realiz- 
able under  actual  conditions,  must  have  once  occurred. 
Considerable  interest  was  excited  when  the  great  Eng- 
lish physiologist  Huxley  believed  he  had  discovered  in 
the  mud  brought  up  from  the  very  bottom  of  the  sea 
an  albuminoid  substance  which  he  called  "Bathybius 
Haeckelii,"  in  honor  of  the  zealous  German  Darwinist 
Haeckel.  In  this  bathybius  (deep-sea  organism)  one 
fancied  for  a  time  that  the  primordial  ooze,  which  had 
originated  from  inorganic  matter  and  from  which  all 
organisms  might  have  been  evolved,  and  of  which  Oken 

had  been  dreaming,  had  been  discovered.     But  the  more 
is  215 


WORLDS   IN   THE   MAKING 

exact  researches  of  the  chemist  Buchanan  demonstrated 
that  the  albuminoid  substance  in  this  primordial  ooze 
consisted  of  flocks  of  gypsum  precipitated  by  alcohol. 

People  then  had  recourse  to  the  most  fantastic  specula- 
tions. Life,  it  was  argued,  might  possibly  have  had  its 
origin  in  the  incandescent  mass  of  the  interior  of  the 
earth.  ,At  high  temperatures  organic  compounds  of 
cyanogen  and  its  derivatives  might  be  formed  which 
would  be  the  carriers  of  life  (Pfliiger).  There  is,  however, 
little  need  of  our  entering  into  any  of  these  speculations 
until  they  have  been  provided  with  an  experimental 
basis. 

Almost  every  year  the  statement  is  repeated  in 
biological  literature  that  we  have  at  last  succeeded  in 
producing  life  from  dead  matter.  Among  the  most  recent 
assertions  of  this  kind,  the  discovery  claimed  by  Butler- 
Burke  has  provoked  much  comment.  He  asserted  that  he 
had  succeeded,  with  the  aid  of  the  marvellous  substance 
radium,  in  instilling  life  into  lifeless  matter — namely,  a 
solution  of  gelatine.  Criticism  has,  however,  relegated 
this  statement,  like  all  similar  ones,  to  the  realm  of  fairy 
tales. 

We  fully  share  the  opinion  which  the  great  natural 
philosopher  Lord  Kelvin  has  expressed  in  the  following 
words:  "A  very  ancient  speculation,  still  clung  to  by 
many  naturalists  (so  much  so  that  I  have  a  choice  of 
modern  terms  to  quote  in  expressing  it),  supposes  that, 
under  meterological  conditions  very  different  from  the 
present,  dead  matter  may  have  run  together  or  crystallized 
or  fermented  into  ' germs  of  life,'  or  'organic  cells/  or 
'protoplasm.'  But  science  brings  a  vast  mass  of  induc- 
tive evidence  against  this  hypothesis  of  spontaneous  gen- 
eration. Dead  matter  cannot  become  living  without 
coming  under  the  influence  of  matter  previously  alive. 

216 


LIFE   THROUGH   THE    UNIVERSE 

This  seems  to  me  as  sure  a  teaching  of  science  as  the  law 
of  gravitation." 

Although  the  latter  verdict  may  be  a  little  dogmatic, 
it  yet  demonstrates  how  strongly  many  scientists  feel 
the  necessity  of  finding  another  way  of  solving  the  prob- 
lem. The  so-called  theory  of  panspermia  really  shows 
a  way.  According  to  this  theory  life-giving  seeds  are 
drifting  about  in  space.  They  encounter  the  plane ts7 
and  fill  their  surfaces  with  life  as  soon  as  the  necessary 
conditions  for  the  existence  of  organic  beings  are  estab- 
lished. 

This  view  was  probably  foreshadowed  long  ago.  Defi- 
nite suggestions  in  this  direction  we  find  in  the  writings 
of  the  Frenchman  Sales-Guy  on  de  Montlivault  (1821), 
who  assumed  that  seeds  from  the  moon  had  awakened  the 
first  life  on  the  surface  of  the  earth.  The  German  physi- 
cian H.  E.  Richter  attempted  to  supplement  the  doctrine 
of  Darwin  by  combining  the  conception  of  panspermia 
with  it.  Flammarion's  book  on  the  plurality  of  inhabited 
worlds. suggested  to  Richter  the  idea  that  seeds  had  come 
from  some  other  inhabited  world  to  our  earth.  He  em- 
phasizes the  fact  that  carbon  has  been  found  in  meteorites 
which  move  in  orbits  similar  to  those  of  the  comets  which 
wander  about  in  space ;  and  in  this  carbon  he  sees  the  rests 
of  organic  life.  There  is  no  proof  at  all  for  this  latter 
opinion.  The  carbon  found  in  meteorites  has  never  ex- 
hibited any  trace  of  organic  structure,  and  we  may  well 
imagine  the  carbon — e.g.,  that  which  appears  to  occur  in 
the  sun — to  be  of  inorganic  origin.  Still  more  fantastic  is 
his  idea  that  organisms  floating  high  in  our  atmosphere 
are  caught  by  the  attraction  of  meteorites  flying  past  our 
planet,  and  are  in  this  way  carried  out  into  universal  space 
and  deposited  upon  other  celestial  bodies.  As  the  surface 

of  meteorites  becomes  incandescent  in  their  flight  through 

217 


WORLDS   IN   THE   MAKING 

the  atmosphere,  any  germs  which  they  might  possibly 
have  caught  would  be  destroyed ;  and  if,  in  spite  of  that, 
a  meteorite  should  become  the  conveyor  of  live  germs, 
those  germs  would  be  burned  in  the  atmosphere  of  the 
planet  on  which  they  descended. 

In  one  point,  however,  we  must  agree  with  Richter. 
There  is  logic  in  his  statement  that  "  The  infinite  space  is 
filled  with,  or  (more  correctly)  contains,  growing,  mature, 
and  dying  celestial  bodies.  By  mature  worlds  we  under- 
stand those  which  are  capable  of  sustaining  organic  life. 
We  regard  the  existence  of  organic  life  in  the  universe 
as  eternal.  Life  has  always  been  there;  it  has  always 
propagated  itself  in  the  shape  of  living  organisms,  from 
cells  and  from  individuals  composed  of  cells."  Man  used 
to  speculate  on  the  origin  of  matter,  but  gave  that  up 
when  experience  taught  him  that  matter  is  indestructible 
and  can  only  be  transformed.  For  similar  reasons  we 
never  inquire  into  the  origin  of  the  energy  of  motion. 
And  we  may  become  accustomed  to  the  idea  that  life  is 
eternal,  and  hence  that  it  is  useless  to  inquire  into  its 
origin. 

The  ideas  of  Richter  were  taken  up  again  in  a  popular 
lecture  delivered  in  1872  by  the  famous  botanist  Ferdinand 
Cohn.  The  best-known  expression  of  opinion  on  the  sub- 
ject, however,  is  that  of  Sir  William  Thomson  (later  Lord 
Kelvin)  in  his  presidential  address  to  the  British  Associa- 
tion at  Edinburgh  in  1871: 

"When  two  great  masses  come  into  collision  in  space, 
it  is  certain  that  a  large  part  of  each  is  melted;  but  it 
seems  also  quite  certain  that  in  many  cases  a  large  quan- 
tity of  debris  must  be  shot  forth  in  all  directions,  much  of 
which  may  have  experienced  no  greater  violence  than 
individual  pieces  of  rock  experience  in  a  landslip  or  in 
blasting  by  gunpowder.  Should  the  time  when  this 

218 


LIFE   THROUGH   THE   UNIVERSE 

earth  comes  into  collision  with  another  body,  comparable 
in  dimensions  to  itself,  be  when  it  is  still  clothed  as  at 
present  with  vegetation,  many  great  and  small  fragments 
carrying  seed  and  living  plants  and  animals  would  un- 
doubtedly be  scattered  through  space.  Hence,  and  be- 
cause we  all  confidently  believe  that  there  are  at  present, 
and  have  been  from  time  immemorial,  many  worlds  of 
life  besides  our  own,  we  must  regard  it  as  probable  in 
the  highest  degree  that  there  are  countless  seed-bearing 
meteoric  stones  moving  about  through  space.  If  at  the 
present  instant  no  life  existed  upon  this  earth,  one  such 
stone  falling  upon  it  might,  by  what  we  blindly  call 
natural  causes,  lead  to  its  becoming  covered  with  vege- 
tation. I  am  fully  conscious  of  the  many  objections 
which  may  be  urged  against  this  hypothesis.  I  will 
not  tax  your  patience  further  by  discussing  any  of  them 
on  the  present  occasion.  All  I  maintain  is  that  I  believe 
them  to  be  all  answerable." 

Unfortunately  we  cannot  share  Lord  Kelvin's  optimism 
regarding  this  point.  It  is,  in  the  first  instance,  ques- 
tionable whether  living  beings  would  be  able  to  survive 
the  violent  impact  of  the  collision  of  two  worlds.  We 
know,  further,  that  the  meteorite  in  its  fall  towards  the 
earth  becomes  incandescent  all  over  its  surface,  and  any 
seeds  on  it  would  therefore  be  deprived  of  their  germi- 
nating power.  Meteorites,  moreover,  show  quite  a  differ- 
ent composition  from  that  of  the  fragments  from  the  sur- 
face of  the  earth  or  a  similar  planet.  Plants  develop 
almost  exclusively  in  loose  soil,  and  a  lump  of  earth  fall- 
ing through  our  atmosphere  would,  no  doubt,  be  disin- 
tegrated into  a  shower  of  small  particles  by  the  resistance 
of  the  atmosphere.  Each  of  these  particles  would  by 
itself  flash  up  like  a  shooting-star,  and  could  not  reach 
the  earth  in  any  other  shape  than  that  of  burned  dust. 

219 


WORLDS   IN  THE  MAKING 

Another  difficulty  is  that  such  collisions,  which,  as  we 
presume,  are  responsible  for  the  flashing-up  of  so-called 
new  stars,  are  rather  rare  phenomena,  so  that  little  likeli- 
hood remains  of  small  seeds  being  transported  to  our  earth 
in  this  manner. 

The  question  has,  however,  entered  into  a  far  more 
favorable  stage  since  the  effects  of  radiation  have  become 
understood. 

Bodies  which,  according  to  the  deductions  of  Schwarz- 
schild,  would  undergo  the  strongest  influence  of  solar  ra- 
diation must  have  a  diameter  of  0.00016  mm.,  supposing 
them  to  be  spherical.  The  first  question  is,  therefore: 
are  there  any  living  seeds  of  such  extraordinary  minute- 
ness? The  repty  of  the  botanist  is  that  the  so-called 
permanent  spores  of  many  bacteria  have  a  size  of  0.0003 
or  0.0002  mm.,  and  there  are,  no  doubt,  much  smaller 
germs  which  our  microscopes  fail  to  disclose.  Thus, 
yellow-fever  in  man,  rabies  in  dogs,  the  foot-and-mouth 
disease  in  cattle,  and  the  so-called  mosaic  disease — com- 
mon to  the  tobacco  plant  in  Netherlandish  India,  and 
also  observed  in  other  countries — are,  no  doubt,  para- 
sitical diseases ;  but  the  respective  parasites  have  not  yet 
been  discovered,  presumably  because  they  are  too  minute 
to  be  visible  under  the  microscope.1 

It  is,  therefore,  very  probable  that  there  are  organisms 
so  small  that  the  radiation  pressure  of  a  sun  would  push 
them  out  into  space,  where  they  might  give  rise  to  life 
on  planets,  provided  they  met  with  favorable  conditions 
for  their  development. 

We  will,  in  the  first  instance,  make  a  rough  calculation 

1  Meanwhile  a  large  number  of  organisms  which  are  invisible  under 
the  ordinary  microscope  have  been  rendered  visible  by  the  aid  of  the 
ultra-microscope,  among  others  the  presumable  microbe  of  the  foot- 
and-mouth  disease. 

220 


LIFE   THROUGH   THE   UNIVERSE 

of  what  would  happen  if  such  an  organism  were  detached 
from  the  earth  and  pushed  out  into  space  by  the  radiation 
pressure  of  our  sun.  The  organism  would,  first  of  all, 
have  to  cross  the  orbit  of  Mars;  then  the  orbits  of  the 
smaller  and  of  the  outer  planets;  and,  having  passed  the 
last  station  of  our  solar  system,  the  orbit  of  Neptune, 
it  would  drift  farther  into  infinite  space  towards  other 
solar  systems.  It  is  not  so  difficult  to  estimate  the  time 
which  the  smallest  particles  would  require  for  this  journey. 
Let  their  specific  gravity  be  that  of  water,  which  will 
very  fairly  correspond  to  the  facts.  The  organisms 
would  cross  the  orbit  of  Mars  after  twenty  days,  the  Jupiter 
orbit  after  eighty  days,  and  the  orbit  of  Neptune  after 
fourteen  months.  Our  nearest  solar  system,  Alpha  Cen- 
tauri,  would  be  reached  in  nine  thousand  years.  These 
calculations  have  been  made  under  the  supposition  that 
the  radiation  pressure  is  four  times  as  strong  as  gravita- 
tion, which  would  be  nearly  correct  according  to  the 
figures  of  Schwarzschild.1 

These  time  intervals  required  for  the  organisms  to  reach 
the  different  planets  of  our  solar  system  are  not  too  long 
for  the  germs  in  question  to  preserve  their  germinating 
power.  The  estimate  is  more  unfavorable  in  the  case 
of  their  transference  from  one  planetary  system  to  an- 
other, which  will  require  thousands  of  years.  But  we 
shall  see  further  on  that  the  very  low  temperature  of 
those  parts  of  space  (about  —220°  C.)  would  suspend  the 
extinction  of  the  germinating  power,  as  it  arrests  all  chem- 
ical reactions. 

As  regards  the  period  during  which  the  germinating 

1  The  radiation  pressure  has  here  been  assumed  to  be  somewhat 
greater  than  on  page  103,  because  the  spores  are  here  regarded  as 
opaque,  while  the  drops  of  hydrocarbons  have  been  regarded  as 
partially  translucid  to  luminous  rays. 

221 


WORLDS    IN    THE   MAKING 


power  can  be  preserved  at  ordinary  temperature,  we 
have  been  told  that  the  so-called  "  mummy  wheat "  which 
had  been  found  in  ancient  Egyptian  tombs  was  still  capa- 
ble of  germination. '  Critics,  however,  have  established 
that  the  respective  statements  of  the  Arabs  concerning 
the  sources  of  that  wheat  are  very  doubtful.  The  French 
scientist  Baudoin  asserts  that  bacteria  capable  of  germina- 
tion were  found  in  a  Roman  tomb  which  had  certainly 
remained  untouched  for  eighteen  hundred  years;  but 
this  statement  is  to  be  received  with  caution.  It  is  cer- 
tain, however,  that  both  seeds  of  some  higher  plants  and 
spores  of  certain  bacteria — e.g.,  anthrax — do  maintain 
their  germinating  power  for  several  years  (say,  twenty), 
and  thus  for  periods  which  are  much  longer  than  those 
we  have  estimated  as  necessary  for  their  transference  to 
our  planet. 

On  the  road  from  the  earth  the  germs  would  for  about 
a  month  be  exposed  to  the  powerful  light  of  the  sun, 
and  it  has  been  demonstrated  that  the  most  highly  re- 
frangible rays  of  the  sun  can  kill  bacteria  and  their  spores 
in  relatively  short  periods.  As  a  rule,  however,  these 
experiments  have  been  conducted  in  such  a  manner  that 
the  spores  could  germinate  on  the  moist  surface  on  which 
they  were  deposited  (for  instance,  in  Marshall  Ward's 
experiments).  That,  however,  does  not  at  all  conform 
to  the  conditions  prevailing  in  planetary  space.  For 
Roux  has  shown  that  anthrax  spores,  which  are  readily 
killed  by  light  when  the  air  has  access,  remain  alive  when 
the  air  is  excluded.  Some  spores  do  not  suffer  from  in- 
sulation at  all.  That  applies,  for  instance,  according  to 
Duclaux,  to  Thyrothrix  scaber,  which  occurs  in  milk  and 
which  may  live  for  a  full  month  under  the  intense  light 
of  the  sun.  All  the  botanists  that  I  have  been  able  to 
consult  are  of  the  opinion  that  we  can  by  no  means  as- 

222 


LIFE   THROUGH   THE    UNIVERSE 

sert  with  certainty  that  spores  would  be  killed  by  the 
light  rays  in  wandering  through  infinite  space. 

It  may  further  be  argued  that  the  spores,  in  their 
journey  through  universal  space,  would  be  exposed  dur- 
ing most  of  that  period  to  an  extreme  cold  which  possibly 
they  might  not  be  able  to  endure.  When  the  spores 
have  passed  the  orbit  of  Neptune,  their  temperature  will 
have  sunk  to  —  220°,  and  farther  out  it  will  sink  still 
lower.  In  recent  years  experiments  have  been  made  in 
the  Jenner  Institute,  in  London,  with  spores  of  bacteria 
which  were  kept  for  twenty  hours  at  a  temperature  of 

—  252°  in  liquid  hydrogen.    Their  germinating  power  was 
not  destroyed  thereby. 

Professor  Macfadyen  has,  indeed,  gone  still  further. 
He  has  demonstrated  that  micro-organisms  may  be  kept 
in  liquid  air  (at  —200°)  for  six  months  without  being 
deprived  of  their  germinating  power.  According  to  what 
I  was  told  on  the  occasion  of  my  last  visit  to  London, 
further  experiments,  continued  for  still  longer  periods, 
have  only  confirmed  this  observation. 

There  is  nothing  improbable  in  the  idea  that  the  ger- 
minating power  should  be  preserved  at  lower  tempera- 
tures for  longer  periods  than  at  our  ordinary  tempera- 
tures. The  loss  of  germinating  power  is  no  doubt  due 
to  some  chemical  process,  and  all  chemical  processes 
proceed  at  slower  rates  at  lower  temperatures  than  they 
do  at  higher.  The  vital  functions  are  intensified  in  the 
ratio  of  1  :  2.5  when  the  temperature  is  raised  by  10°  C. 
(18°  F.).  By  the  time  that  the  spores  reached  the  orbit 
of  Neptune  and  their  temperature  had  been  lowered  to 

—  220°,  their  vital  energy  would,  according  to  this  ratio, 
react  with  one  thousand  millions  less  intensity  than  at  10°. 
The  germinating  power  of  the  spores  would  hence,  at 

-  220°,  during  the  period  of  three  million  years,  not  be 

223 


WORLDS    IN   THE   MAKING 

diminished  to  any  greater  degree  than  during  one  day  at 
10°.  It  is,  therefore,  not  at  all  unreasonable  to  assert  that 
the  intense  cold  of  space  will  act  like  a  most  effective 
preservative  upon  the  seeds,  and  that  they  will  in  conse- 
quence be  able  to  endure  much  longer  journeys  than  we 
could  assume  if  we  judged  from  their  behavior  at  or- 
dinary temperatures. 

It  is  similar  with  the  drying  effect  which  may  be  so  in- 
jurious to  plant  life.  In  interplanetary  space,  which  is 
devoid  of  atmosphere,  absolute  dryness  prevails.  An 
investigation  by  B.  Schrober  demonstrates  that  the  green 
alga  Pleurococcus  vulgaris,  which  is  so  common  on  the 
trunks  of  trees,  can  be  kept  in  absolute  dryness  (over 
concentrated  sulphuric  acid  in  a  desiccator)  for  twenty 
weeks  without  being  killed.  Seeds  and  spores  may  last 
still  longer  in  a  dry  atmosphere. 

Now,  the  tension  of  water  vapor  decreases  in  nearly 
the  same  ratio  as  the  speed  of  the  reaction  with  lower 
temperatures.  The  evaporation  of  water — i.  e.,  the  dry- 
ing effect — may  hence,  at  a  temperature  of  —220°,  not 
proceed  further  in  three  million  years  than  it  will  in 
one  day  at  10°.  We  have  thus  several  plausible  reasons 
for  concluding  that  spores  which  oppose  an  effective  re- 
sistance to  drying  may  well  be  carried  from  one  planet 
to  another  and  from  one  planetary  system  to  another 
without  sacrificing  their  vital  energy. 

The  destructive  effect  of  light  is,  according  to  the  ex- 
periments of  Roux,  no  doubt  due  to  the  fact  that  the 
rays  of  light  call  forth  an  oxidation  by  the  intermediation 
of  the  surrounding  air.  This  possibility  is  excluded  in 
interplanetary  space.  Moreover,  the  radiation  of  the  sun 
is  nine  hundred  times  fainter  in  the  orbit  of  Neptune 
than  in  the  orbit  of  the  earth,  and  half-way  to  the  near- 
est fixed  star,  Alpha  Centauri,  twenty  million  times 

224 


LIFE   THROUGH   THE    UNIVERSE 

feebler.    Light,  therefore,  will  not  do  much  harm  to  the 
spores  during  their  transference. 

If,  therefore,  spores  of  the  most  minute  organisms 
could  escape  from  the  earth,  they  might  travel  in  all 
directions,  and  the  whole  universe  might,  so  to  say,  be 
sown  with  them.  But  now  comes  the  question:  how 
can  they  escape  from  the  earth  against  the  effect  of 
gravitation?  Corpuscles  of  such  small  weight  would 
naturally  be  carried  away  by  any  aerial  current.  A  small 
rain -drop,  ^V  mm.  in  diameter,  falls,  at  ordinary  air 
pressure,  about  4  cm.  per  second.  We  can  calculate  from 
this  observation  that  a  bacteria  spore  0.00016  mm.  in 
diameter  would  only  fall  83  m.  in  the  course  of  a  year. 
It  is  obvious  that  particles  of  this  minuteness  would  be 
swept  away  by  every  air  current  they  met  until  they 
reached  the  most  diluted  air  of  the  highest  strata.  An 
air  current  of  a  velocity  of  2  m.  per  second  would  take 
them  to  a  height  where  the  air  pressure  is  only  0.001 
mm. — i.e.,  to  a  height  of  about  100 km.  (60  miles).  But 
the  air  currents  can  never  push  the  particle  outside  of 
our  atmosphere. 

In  order  to  raise  the  spores  to  still  higher  levels  we 
must  have  recourse  to  other  forces,  and  we  know  that 
electrical  forces  can  help  us  out  of  almost  any  difficulty. 
At  heights  of  100  km.  the  phenomena  of  the  radiating 
aurora  take  place.  We  believe  that  the  aurorse  are 
produced  by  the  discharge  of  large  quantities  of  nega- 
tively charged  dust  coming  from  the  sun.  If,  therefore, 
the  spore  in  question  should  take  up  negative  electricity 
from  the  solar  dust  during  an  electric  discharge,  it  may 
be  driven  out  into  the  sea  of  ether  by  the  repulsive  charges 
of  the  other  particles. 

-We  suppose,   now,   that  the  electrical   charges — like 
matter — cannot  be  subdivided  without  limit.    We  must 

225 


WORLDS    IN    THE    MAKING 

finally  come  to  a  minimum  charge,  and  this  charge  has 
been  calculated  at  about  3.5.10""10  electrostatic  unit. 

We  can,  without  difficulty,  calculate  the  intensity  of 
an  electric  field  capable  of  urging  the  charged  spore  of 
0.00016  mm.  upward  against  the  force  of  gravity.  The 
required  field-strength  is  only  200  volts  per  metre.  Such 
fields  are  often  observed  on  the  surface  of  the  earth  with 
a  clear  sky,  and  they  are,  indeed,  almost  normal.  The 
electric  field  of  a  region  in  which  an  auroral  display  takes 
place  is  probably  much  more  intense,  and  would,  without 
doubt,  be  of  sufficient  intensity  to  urge  the  small  elec- 
trically charged  spores  which  convection  currents  had 
carried  up  to  these  strata,  farther  out  into  space  against 
the  force  of  gravity. 

It  is  thus  probable  that  germs  of  the  lowest  organisms 
known  to  us  are  continually  being  carried  away  from 
the  earth  and  the  other  planets  upon  which  they  exist. 
As  seeds  in  general,  so  most  of  these  spores,  thus  carried 
away,  will  no  doubt  meet  death  in  the  cold  infinite 
space  of  the  universe.  Yet  a  small  number  of  spores 
will  fall  on  some  other  world,  and  may  there  be  able  to 
spread  life  if  the  conditions  be  suitable.  In  many  cases 
conditions  will  not  be  suitable.  Occasionally,  however, 
the  spores  will  fall  on  favorable  soil.  It  may  take  one 
million  or  several  millions  of  years  from  the  age  at  which 
a  planet  could  possibly  begin  to  sustain  life  to  the  time 
when  the  first  seed  falls  upon  it  and  germinates,  and 
when  organic  life  is  thus  originated.  This  period  is  of 
little  significance  in  comparison  with  the  time  during 
which  life  will  afterwards  flourish  on  the  planet.' 

The  germs  which  in  this  way  escape  from  the  planets 
on  which  their  ancestors  had  found  abode,  may  either 
wander  unobstructed  through  space,  or  they  may,  as 
we  have  indicated,  reach  outer  planet?,  or  planets  moving 

220 


LIFE   THROUGH   THE   UNIVERSE 

about  other  suns,  or  they  may  meet  with  larger  particles 
of  dust  rushing  towards  the  sun. 

We  have  spoken  of  the  Zodiacal  Light  and  that  part  of 
it  which  has  been  designated  the  counter-glow.  This 
latter  glow  is  regularly  seen  in  the  tropics  and  occasion- 
ally in  that  portion  of  our  heavens  which  is  just  oppo- 
site the  sun.  Astronomers  ascribe  the  counter-glow  to 
streams  of  fine  dust  which  are  drawn  towards  the  sun 
(compare  page  147).  Let  us  assume  that  a  seed  of  the 
diameter  of  0.00016  mm.  strikes  against  a  grain  of  dust 
which  is  a  thousand  times  as  large  (0.0016  mm.  diameter), 
and  attaches  itself  to  its  surface.  This  spore  will  be  car- 
ried by  the  grain  of  dust  towards  the  sun;  it  will  cross 
the  orbits  of  the  inner  planets,  and  it  may  descend  in 
their  atmospheres.  Those  grains  of  dust  do  not,  by  any 
means,  require  very  long  spaces  of  time  to  pass  from 
one  planetary  orbit  to  another.  If  we  assume  that  the 
spore  starts  with  zero  velocity  near  Neptune  (in  which 
case  the  seed  might  originate  from  the  moon  of  Neptune; 
for  Neptune  itself,  like  Uranus,  Saturn,  and  Jupiter, 
is  not  yet  sufficiently  cooled  to  sustain  life),  the  spore 
would  reach  the  orbit  of  Uranus  in  twenty-one  years,  and 
of  Mercury  in  twenty-nine  years.  With  the  same  initial 
velocity  such  particles  would  be  twelve  years  in  passing 
between  the  orbits  of  Uranus  and  Saturn,  four  years  be- 
tween Saturn  and  Jupiter,  two  years  between  Jupiter  and 
Mars,  eighty-four  days  between  Mars  and  the  earth,  forty 
days  between  the  earth  and  Venus,  and  twenty-eight  days 
between  Venus  and  Mercury. 

We  see  from  these  time  estimates  that  the  germs,  to- 
gether with  the  grains  of  dust  to  which  they  have  at- 
tached themselves,  might  move  towards  the  sun  with 
much  smaller  velocity  (from  ten  to  twenty  times  smaller) 
without  our  having  to  fear  any  loss  of  their  germinating 

227 


WORLDS    IN   THE   MAKING 

powers  during  the  transit.  In  other  words,  if  these  seeds 
adhere  to  the  particles,  ninety  or  ninety -five  per  cent,  of 
whose  weight  is  balanced  by  the  radiation  pressure,  they 
may  soon  fall  into  the  atmosphere  of  some  inner  planet 
with  the  moderate  velocity  of  a  few  kilometres  per  second. 
It  is  easy  to  calculate  that  if  such  a  particle  should,  in 
falling,  be  arrested  in  its  motion  after  the  first  second,  it 
would  yet,  thanks  to  the  strong  heat  radiation  from  it,  not 
be  heated  by  more  than  100°  Cent.  (212°  F.)  above  the 
temperature  of  its  surroundings.  Such  a  temperature 
can  be  borne  by  the  spores  of  bacteria  without  fatal 
effects  for  much  more  than  one  second.  After  the  par- 
ticles, together  with  the  seed  adhering  to  them,  have 
once  been  stopped,  they  will  slowly  descend,  or  will  be 
carried  down  to  the  surface  of  the  nearest  planet  by  de- 
scending convection  currents. 

In  this  way  life  would  be  transferred  from  one  point  of 
a  planetary  system,  on  which  it  had  taken  root,  to  other 
locations  in  the  same  planetary  system  which  favor  the 
development  of  life. 

The  seeds  not  caught  by  such  particles  of  dust  may 
be  taken  over  to  other  solar  systems,  and  finally  be 
stopped  by  the  radiation  pressure  of  their  suns.  They 
cannot  penetrate  any  farther  than  to  spots  at  which  the 
radiation  pressure  is  as  strong  as  at  their  starting-points. 
Consequently,  germs  from  the  earth,  which  is  five  times 
as  near  the  sun  as  Jupiter,  could  approach  another  sun 
within  a  fifth  of  the  distance  at  which  germs  from  Jupiter 
would  be  stopped. 

'  Somewhere  near  the  suns,  where  the  seeds  are  arrested 
by  the  radiation  pressure  to  be  turned  back  into  space, 
there  will  evidently  be  accumulations  of  these  seeds.  The 
planets  which  circulate  around  their  suns  have  therefore 
more  chance  of  meeting  them  than  if  they  were  not  in 

228 


LIFE   THROUGH    THE    UNIVERSE 

the  vicinity  of  a  sun.  The  germs  will  have  lost  the  great 
velocity  with  which  they  wandered  from  one  solar  system 
to  another,  and  they  will  not  be  heated  so  greatly  in  fall- 
ing through  the  atmospheres  of  the  planets  which  they 
meet. 

The  seeds  which  are  turned  back  into  space  when 
coming  near  a  sun  will  there  perhaps  meet  with  particles 
whose  weight  is  somewhat  greater  than  the  repelling 
power  of  the  radiation  pressure.  They  would,  therefore, 
turn  back  to  the  suns.  Like  the  germs,  and  for  similar 
reasons,  these  particles  would  consequently  be  concen- 
trated about  the  sun.  The  small  seeds  have,  therefore, 
a  comparatively  better  chance  of  being  arrested  before 
their  return  to  space  by  contact  with  such  particles, 
and  of  being  carried  to  the  planets  near  that  sun. 

In  this  manner  life  may  have  been  transplanted  for 
eternal  ages  from  solar  system  to  solar  system  and  from 
planet  to  planet  of  the  same  system.  But  as  among  the 
billions  of  grains  of  pollen  which  the  wind  carries  away 
from  a  large  tree — a  fir-tree,  for  instance — only  one  may 
on  an  average  give  birth  to  a  new  tree,  thus  of  the 
billions,  or  perhaps  trillions,  of  germs  which  the  radiation 
pressure  drives  out  into  space,  only  one  may  really  bring 
life  to  a  foreign  planet  on  which  life  had  not  yet  arisen, 
and  become  the  originator  of  living  beings  on  that  planet. 

Finally,  we  perceive  that,  according  to  this  version  of 
the  theory  of  panspermia,  all  organic  beings  in  the  whole 
universe  should  be  related  to  one  another,  and  should 
consist  of  cells  which  are  built  up  of  carbon,  hydrogen, 
oxygen,  and  nitrogen.  The  imagined  existence  of  living 
beings  in  other  worlds  in  whose  constitution  carbon  is 
supposed  to  be  replaced  by  silicon  or  titanium  must  be 
relegated  to  the  realm  of  improbability.  Life  on  other 
inhabited  planets  has  probably  developed  along  lines 

229 


WORLDS   IN   THE   MAKING 

which  are  closely  related  to  those  of  our  earth,  and  this 
implies  the  conclusion  that  life  must  always  recommence 
from  its  very  lowest  type,  just  as  every  individual,  how- 
ever highly  developed  it  may  be,  has  by  itself  passed 
through  all  the  stages  of  evolution  from  the  single  cell 
upward. 

All  these  conclusions  are  in  beautiful  harmony  with 
the  general  properties  characteristic  of  life  on  our  earth. 
It  cannot  be  denied  that  this  interpretation  of  the  theory 
of  panspermia  is  distinguished  by  perfect  consistency, 
which  is  the  most  important  criterion  of  the  probability 
of  a  cosmogonical  theory. 

There  is  little  probability,  though,  of  our  ever  being 
able  to  demonstrate  the  correctness  of  this  view  by  an 
examination  of  seeds  falling  down  upon  our  earth.  For 
the  number  of  germs  which  reach  us  from  other  worlds 
will  be  extremely  limited — not  more,  perhaps,  than  a 
few  within  a  year  all  over  the  earth's  surface ;  and  those, 
moreover,  will  presumably  strongly  resemble  the  single- 
cell  spores  with  which  the  winds  play  in  our  atmosphere. 
It  would  be  difficult,  if  not  impossible,  to  prove  the 
celestial  origin  of  any  such  germs  if  they  should  be  found 
contrary  to  our  assumption. 


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